test_branching <- function(list_years){

    return(list(list(paste0(list_years,"/xAclassif"),
             paste0(list_years,"/xAconf")), 
             list(paste0(list_years,"/xBclassif"),
             paste0(list_years,"/xBconf"))))
}

tar_plan(
    
   tar_target(list_years,
          find_years("./data_folder")),

    tar_target(list_tiles,
               test_branching(list_years),
               pattern=map(list_years),
              iteration="list"),

    tar_target(tile_pipeline,
               list_tiles,
               pattern=map(list_tiles),
               iteration="list"),
    
    tar_target(tile_pipeline_disagregated,
               tile_pipeline,
               pattern=map(tile_pipeline),
               iteration="list"),
)

List of 1
 $ tile_pipeline_4ccfb1d61e014683:List of 2
  ..$ :List of 2
  .. ..$ : chr "./data_folder/years/2017/xAclassif"
  .. ..$ : chr "./data_folder/years/2017/xAconf"
  ..$ :List of 2
  .. ..$ : chr "./data_folder/years/2017/xBclassif"
  .. ..$ : chr "./data_folder/years/2017/xBconf"

List of 1
 $ tile_pipeline_disagregated_abcb8e0e83598115:List of 2
  ..$ :List of 2
  .. ..$ : chr "./data_folder/years/2017/xAclassif"
  .. ..$ : chr "./data_folder/years/2017/xAconf"
  ..$ :List of 2
  .. ..$ : chr "./data_folder/years/2017/xBclassif"
  .. ..$ : chr "./data_folder/years/2017/xBconf"

Level 1 => 2017
                  Level 2 => 2017 contain 2 couple of 2 files => list (list( xAclassif, xAconf), list( xBclassif, xBconf))
                                Level 3  => apply target on folder 2017 on list( xAclassif, xAconf)
                                Level 3 => apply target on folder 2017 on list( xBclassif, xBconf)  
                   Level 2 => 2018 contain 2 couple of 2 files => list (list( xAclassif, xAconf), list( xBclassif, xBconf))
                                 Level 3  => apply target on folder 2018on list( xAclassif, xAconf)
                                  Level 3 => apply target on folder 2018 on list( xBclassif, xBconf)