TM 10-5430-243-12&P 0034 00 0034 00-3 Change 1 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 items source coded “XA” or those   aircraft support items restricted by requirements of AR 750-1. 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: Third position.  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 the following levels of maintenance: Maintenance Code Application/Explanation C  –   Crew or operator maintenance done within unit/AVUM maintenance. O  – Unit level/AVUM maintenance can remove, replace, and use the item. F  – Direct support/AVIM maintenance can remove, replace, and use the item. H  – General support maintenance can remove, replace, and use the item. L  – Specialized repair activity can remove, replace, and use the item. D  – Depot can remove, replace, and use the item. Fourth position.  The maintenance code entered in the fourth position tells you whether or not the item is to be repaired and identifies the lowest maintenance level with the capability to do complete repair (perform all authorized repair functions). NOTE Some limited repair may be done on the item at a lower level of maintenance, if authorized by the Maintenance Allocation Chart (MAC) and SMR codes. Maintenance Code Application/Explanation O  –   Unit/AVUM is the lowest level that can do complete repair of the item. F  – Direct support/AVIM is the lowest level that can do complete repair of the item. H  – General support is the lowest level that can do complete repair of the item. L  – Specialized repair activity is the lowest level that can do complete repair of the item. D  – Depot is the lowest level that can do complete repair of the item. Z  – Nonreparable.  No repair is authorized. B  – No repair is authorized.  No parts or special tools are authorized for maintenance of “B” coded item.  However, the item may be reconditioned by adjusting, lubricating, etc., at the user level.