Can we limit/assign order reason per sales document type?
e.g. Order Type OR => will only see reasons 01 to 04 in the dropdown list
Order Type RE = will only display reasons 05 to 10 in the dropdown list
As suggested already you need to do the enhancement in the USEREXIT_MOVE_FIELD_TO_VBAK. In addition you need to add a Z table and define the allowed order reasons for the sales order type. The validation would be based on the entries in this table.