A compatibility group letter is a designated alphabetical letter used to categorize different types of explosives for purposes of stowage and segregation.
The compatibility group letters are used to prevent certain types of explosives, that when combined could be extremely dangerous, from being transported and stored together.
There are 13 compatibility group letters, A through H, J through L, N, and S. A description of each can be found in 173.52(b) Table 1.
The compatibility group letter for an explosive must be displayed on placards when the shipment is by aircraft or vessel. If the shipment if by highway or rail, the compatibility group letter is not required to be displayed on placards according to 172.504(g).
A classification code, such as 1.4S, consists of the division number (1.4) and the compatibility group letter (S). The Associate Administrator for Hazardous Materials Safety assigns the classification code for an explosive. There are 35 possible classification codes for explosives. The classification codes and compatibility groups for explosives are specified in the tables found in 173.52(b).
See Explosives Compatibility for information on loading, transporting, and storing explosives.