If you generate mapping from a work item type instance used in a subflow, but you do not need to map all of that instance’s fields to fields in the Work Item Type Instances table, you can easily eliminate the mapping for any un-needed inherited field.
To delete a field mapping, in the Inherited Fields table, right-click the field whose mapping you want to delete, and click Delete Mapping in the shortcut menu.
In the Inherited Fields table, notice that the field whose mapping you deleted now indicates < Not Mapped > in the Mapping To Local Instance column, and in that same column, the work item type instance for that field now indicates < Partially Mapped >, meaning that only some of its fields are currently mapped to a work item type instance.