ARMY TM 9-6115-639-23P AIR FORCE TO 35C2-3-386-54 MARINE CORPS TM 10155A-23P/2A AF   (Assembled by requisitioned or fabricated and assembled at the category of mainten- DS category) ance indicated by the source code.  If the 3rd position code of the SMR AH   (Assembled by authorizes you to replace the item, but the source code indicates the   GS Category) item  is assembled at a higher category, order the item from the AL   (Assembled by SRA) higher category of maintenance. AD   (Assembled by Depot) Code Explanation XA -- Do not requisition a “XA”-coded item. Order its next higher assembly. (Also, refer to the NOTE below.) XB -- If an “XB” item is not available from salvage, order it using the FSCM and part number given. XC -- Installation drawing, diagram, instruction sheet, field service drawing, that is identified by manufacturer's part number. XD -- Item is not stocked.  Order a “XD”-coded item through normal supply channels using the FSCM and part number given, if no NSN is available. NOTE:  Cannibalization or controlled exchange, when authorized, may be used as a source   of supply for items with the above source codes, except for those source coded “XA” or those aircraft support items restricted by requirements of AR 700-42. (2) Maintenance  Code.    Maintenance  codes  tell  you  the  category  (s)  of  maintenance  authorized  to USE and REPAIR support items.  The  maintenance codes are  entered in the 3rd and 4th positions of the SMR Code as follows: (a)    The maintenance code entered in the 3rd position tells you the lowest maintenance category authorized to remove, replace, and use an item.  The maintenance code entered in the 3rd position will indicate authorization to one of the following categorys of maintenance. 3