TM 9-2815-250-24&P C-3. EXPLANATION OF COLUMNS (SECTIONS II AND III) (continued). NOTE Cannibalization or controlled exchange, when authorized, may be used as a source of  supply  for  items  with  the  preceding  source  codes,  except  for  those  source- coded “XA”. XA - DO NOT requisition an "XA"-coded item.  Order its next higher assembly. XB - If an "XB" item is not available from salvage, order it using the CAGEC and part number given. XC - Installation  drawing,  diagram,  instruction  sheet,  or  field  service  drawing  that  is  identified  by  manufacturer's  part number. XD - Item  is  not  stocked.    Order  an  "XD"-coded  item  through  normal  supply  channels  using  the  CAGEC  and  part number given if no NSN is available. (2) Maintenance Code.  Maintenance codes tell you the level(s) of maintenance authorized to use and repair support  items.    The  maintenance  codes  are  entered  in  the  third  and  fourth  positions  of  the  SMR  code  as follows: (a) The maintenance code entered in the third position tells you the lowest maintenance level authorized to remove, replace, and use an item.  The maintenance code entered in the third position will indicate authorization to one of the following levels of maintenance: Code C O F H L D Application/Explanation Crew or Operator maintenance done within Unit maintenance or Aviation Unit maintenance. Unit maintenance or Aviation Unit can remove, replace, and use the item. Direct  Support  or  Aviation  Intermediate  level  can  remove,  replace,  and  use the item. General Support level can remove, replace, and use the item. Specialized repair activity (SRA) can remove, replace, and use the item. Depot level can remove, replace, and use the item. NOTE If  authorized  by  the  maintenance  allocation  chart  (MAC)  and  SMR  codes,  some limited repair may be done on an item at a lower level of maintenance. (b) The  maintenance  code  entered  in  the  fourth  position  tells  whether  the  item  is  to  be  repaired  and identifies  the  lowest  maintenance  level  with  the  capability  to  do  complete  repair  (i.e.,  perform  all authorized "Repair" functions).  This position will contain one of the following maintenance codes: C-3