TM 10-4930-248-13&P AO  -  (Assembled    by Unit/AVUM Items   with   these   codes  are  not  to  be  requested/requisitioned Level) individually. The  parts   that   make   up  the   assembled     item   must   be AF  -   (Assembled    by requisitioned or fabricated and   assembled    at  the  level   of DS/AVIM   Level) maintenance indicated     by  the  source   code.    If the  3rd  position    code  of AH  -(Assembled at   GS t   e code  authorizes you  to  replace    the   item,   but   the   source   code Category) indicates    the  items   are  assembled     at  a higher    level,   order   the  item AL   -(Assembled by from  the  higher    level   of maintenance. SRA) AD  -(Assembled by Depot) XA   - Do not  requisition an  "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  CAGEC   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   an  "XD"-coded    item   through     normal    supply    channels    using   the  CAGEC 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  750-1. (2)     Maintenance Code. Maintenance codes  tell  you  the   level(s)   of the   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. Maintenance Code Application/Explanation C Crew  or  operator    maintenance done  within    unit/AVUM maintenance. 0 Unit   level/AVUM can  remove,   replace, and  use  the  item. F Direct   support/AVIM maintenance can  remove,    replace, and  use  the   item. H General support    level   can   remove,   replace,    and   use  the  item. L Specialized repair   activity can  remove,    replace, and  use  the  item. D Depot   level   can  remove,    replace,    and   use  the   item. I f C-3