diff --git a/spec/data-model/README.md b/spec/data-model/README.md
index 3577f72dbf..f65df5e4a7 100644
--- a/spec/data-model/README.md
+++ b/spec/data-model/README.md
@@ -117,28 +117,33 @@ The `body` strings are the "cooked" _text_ values, i.e. escape sequences are pro
 
 Implementations MUST NOT rely on the set of `Expression` interfaces being exhaustive,
 as future versions of this specification MAY define additional expressions.
-An `Expression` `func` with an unrecognized value SHOULD be treated as an `UnsupportedExpression` value.
 
 ```ts
 interface Pattern {
   body: Array<string | Expression>;
 }
 
-type Expression = LiteralExpression | VariableExpression | FunctionExpression;
+type Expression = LiteralExpression | VariableExpression | FunctionExpression |
+                  UnsupportedExpression;
 
 interface LiteralExpression {
   arg: Literal;
-  func?: FunctionRef | UnsupportedExpression;
+  annotation?: FunctionAnnotation | UnsupportedAnnotation;
 }
 
 interface VariableExpression {
   arg: VariableRef;
-  func?: FunctionRef | UnsupportedExpression;
+  annotation?: FunctionAnnotation | UnsupportedAnnotation;
 }
 
 interface FunctionExpression {
   arg?: never;
-  func: FunctionRef | UnsupportedExpression;
+  annotation: FunctionAnnotation;
+}
+
+interface UnsupportedExpression {
+  arg?: never;
+  annotation: UnsupportedAnnotation;
 }
 ```
 
@@ -166,22 +171,19 @@ interface VariableRef {
 }
 ```
 
-A `FunctionRef` represents an _expression_ with a _function_ _annotation_.
-In a `FunctionRef`,
+A `FunctionAnnotation` represents a _function_ _annotation_.
+In a `FunctionAnnotation`,
 the `kind` corresponds to the starting sigil of a _function_:
 `'open'` for `+`, `'close'` for `-`, and `'value'` for `:`.
 The `name` does not include this starting sigil.
 
-The optional `operand` is the _literal_ or _variable_
-before the _annotation_ in the _expression_, if present.
 Each _option_ is represented by an `Option`.
 
 ```ts
-interface FunctionRef {
+interface FunctionAnnotation {
   type: "function";
   kind: "open" | "close" | "value";
   name: string;
-  operand?: Literal | VariableRef;
   options?: Option[];
 }
 
@@ -191,9 +193,8 @@ interface Option {
 }
 ```
 
-An `UnsupportedExpression` represents an _expression_ with a
-_reserved annotation_ or a _private-use annotation_ not supported
-by the implementation.
+An `UnsupportedAnnotation` represents a
+_private-use annotation_ not supported by the implementation or a _reserved annotation_.
 The `sigil` corresponds to the starting sigil of the _annotation_.
 The `source` is the "raw" value (i.e. escape sequences are not processed)
 and does not include the starting `sigil`.
@@ -204,9 +205,6 @@ and does not include the starting `sigil`.
 > This would result in new interfaces being added to
 > this data model.
 
-If the _expression_ includes a _literal_ or _variable_ before the _annotation_,
-it is included as the `operand`.
-
 When parsing the syntax of a _message_ that includes a _private-use annotation_
 supported by the implementation,
 the implementation SHOULD represent it in the data model
@@ -214,11 +212,10 @@ using an interface appropriate for the semantics and meaning
 that the implementation attaches to that _annotation_.
 
 ```ts
-interface UnsupportedExpression {
-  type: "unsupported-expression";
+interface UnsupportedAnnotation {
+  type: "unsupported-annotation";
   sigil: "!" | "@" | "#" | "%" | "^" | "&" | "*" | "<" | ">" | "/" | "?" | "~";
   source: string;
-  operand?: Literal | VariableRef;
 }
 ```
 
diff --git a/spec/data-model/message.dtd b/spec/data-model/message.dtd
index 5f1810f39b..2a6df759db 100644
--- a/spec/data-model/message.dtd
+++ b/spec/data-model/message.dtd
@@ -24,8 +24,8 @@
 <!ELEMENT pattern (#PCDATA | expression)*>
 
 <!ELEMENT expression (
-  ((literal | variable), (function | unsupportedExpression)?) |
-  function | unsupportedExpression
+  ((literal | variable), (functionAnnotation | unsupportedAnnotation)?) |
+  functionAnnotation | unsupportedAnnotation
 )>
 
 <!ELEMENT literal (#PCDATA)>
@@ -34,13 +34,13 @@
 <!ELEMENT variable (EMPTY)>
 <!ATTLIST variable name NMTOKEN #REQUIRED>
 
-<!ELEMENT function (option)*>
-<!ATTLIST function
+<!ELEMENT functionAnnotation (option)*>
+<!ATTLIST functionAnnotation
   kind (open | close | value) #REQUIRED
   name NMTOKEN #REQUIRED
 >
 <!ELEMENT option (literal | variable)>
 <!ATTLIST option name NMTOKEN #REQUIRED>
 
-<!ELEMENT unsupportedExpression (#PCDATA)>
-<!ATTLIST unsupportedExpression sigil CDATA #REQUIRED>
+<!ELEMENT unsupportedAnnotation (#PCDATA)>
+<!ATTLIST unsupportedAnnotation sigil CDATA #REQUIRED>
diff --git a/spec/data-model/message.json b/spec/data-model/message.json
index 4b6c154ffd..acce03ffcb 100644
--- a/spec/data-model/message.json
+++ b/spec/data-model/message.json
@@ -22,11 +22,18 @@
       },
       "required": ["type", "name"]
     },
-    "value": {
-      "oneOf": [{ "$ref": "#/$defs/literal" }, { "$ref": "#/$defs/variable" }]
+    "option": {
+      "type": "object",
+      "properties": {
+        "name": { "type": "string" },
+        "value": {
+          "oneOf": [{ "$ref": "#/$defs/literal" }, { "$ref": "#/$defs/variable" }]
+        }
+      },
+      "required": ["name", "value"]
     },
 
-    "function": {
+    "function-annotation": {
       "type": "object",
       "properties": {
         "type": { "const": "function" },
@@ -34,24 +41,17 @@
         "name": { "type": "string" },
         "options": {
           "type": "array",
-          "items": {
-            "type": "object",
-            "properties": {
-              "name": { "type": "string" },
-              "value": { "$ref": "#/$defs/value" }
-            },
-            "required": ["name", "value"]
-          }
+          "items": { "$ref": "#/$defs/option" }
         }
       },
       "required": ["type", "kind", "name"]
     },
-    "unsupported-expression": {
+    "unsupported-annotation": {
       "type": "object",
       "properties": {
-        "type": { "const": "unsupported-expression" },
+        "type": { "const": "unsupported-annotation" },
         "sigil": {
-          "enum": ["!", "@", "#", "%", "^", "&", "*", "<", ">", "?", "~"]
+          "enum": ["!", "@", "#", "%", "^", "&", "*", "<", ">", "/", "?", "~"]
         },
         "source": { "type": "string" }
       },
@@ -59,8 +59,8 @@
     },
     "annotation": {
       "oneOf": [
-        { "$ref": "#/$defs/function" },
-        { "$ref": "#/$defs/unsupported-expression" }
+        { "$ref": "#/$defs/function-annotation" },
+        { "$ref": "#/$defs/unsupported-annotation" }
       ]
     },
 
@@ -68,7 +68,7 @@
       "type": "object",
       "properties": {
         "arg": { "$ref": "#/$defs/literal" },
-        "func": { "$ref": "#/$defs/annotation" }
+        "annotation": { "$ref": "#/$defs/annotation" }
       },
       "required": ["arg"]
     },
@@ -76,22 +76,30 @@
       "type": "object",
       "properties": {
         "arg": { "$ref": "#/$defs/variable" },
-        "func": { "$ref": "#/$defs/annotation" }
+        "annotation": { "$ref": "#/$defs/annotation" }
       },
       "required": ["arg"]
     },
     "function-expression": {
       "type": "object",
       "properties": {
-        "func": { "$ref": "#/$defs/annotation" }
+        "annotation": { "$ref": "#/$defs/function-annotation" }
       },
-      "required": ["func"]
+      "required": ["annotation"]
+    },
+    "unsupported-expression": {
+      "type": "object",
+      "properties": {
+        "annotation": { "$ref": "#/$defs/unsupported-annotation" }
+      },
+      "required": ["annotation"]
     },
     "expression": {
       "oneOf": [
         { "$ref": "#/$defs/literal-expression" },
         { "$ref": "#/$defs/variable-expression" },
-        { "$ref": "#/$defs/function-expression" }
+        { "$ref": "#/$defs/function-expression" },
+        { "$ref": "#/$defs/unsupported-expression" }
       ]
     },
 
diff --git a/spec/message.abnf b/spec/message.abnf
index bd7dbeb8eb..8d6950d2cc 100644
--- a/spec/message.abnf
+++ b/spec/message.abnf
@@ -19,13 +19,13 @@ selector = expression
 variant = key *(s key) [s] quoted-pattern
 key = literal / "*"
 
-expression = literal-expression / variable-expression / function-expression
+expression = literal-expression / variable-expression / annotation-expression
 literal-expression = "{" [s] literal [s annotation] [s] "}"
 variable-expression = "{" [s] variable [s annotation] [s] "}"
-function-expression = "{" [s] annotation [s] "}"
+annotation-expression = "{" [s] annotation [s] "}"
 annotation = (function *(s option))
-           / reserved-annotation
            / private-use-annotation
+           / reserved-annotation
 
 literal = quoted / unquoted
 variable = "$" name
diff --git a/spec/syntax.md b/spec/syntax.md
index cf34de5570..e4ce167bb7 100644
--- a/spec/syntax.md
+++ b/spec/syntax.md
@@ -431,13 +431,13 @@ optionally followed by an _annotation_.
 A **_<dfn>variable-expression</dfn>_** contains a _variable_,
 optionally followed by an _annotation_.
 
-A **_<dfn>function-expression</dfn>_** contains only an _annotation_.
+An **_<dfn>annotation-expression</dfn>_** contains an _annotation_ without an _operand_.
 
 ```abnf
-expression = literal-expression / variable-expression / function-expression
+expression = literal-expression / variable-expression / annotation-expression
 literal-expression = "{" [s] literal [s annotation] [s] "}"
 variable-expression = "{" [s] variable [s annotation] [s] "}"
-function-expression = "{" [s] annotation [s] "}"
+annotation-expression = "{" [s] annotation [s] "}"
 ```
 
 There are several types of _expression_ that can appear in a _message_.
@@ -476,12 +476,12 @@ Additionally, an _input-declaration_ can contain a _variable-expression_.
 
 An **_<dfn>annotation</dfn>_** is part of an _expression_ containing either
 a _function_ together with its associated _options_, or
-a _reserved annotation_ or a _private-use annotation_.
+a _private-use annotation_ or a _reserved annotation_.
 
 ```abnf
 annotation = (function *(s option))
-           / reserved-annotation
            / private-use-annotation
+           / reserved-annotation
 ```
 
 An **_<dfn>operand</dfn>_** is the _literal_ of a _literal-expression_ or