From the benchmade.com FAQ:
1. What does the S, BT, SBT, HS, HSSR and the like stand for?
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As for the "HS" and the "HSSR", they designate M2 high-speed steel blades with the first being plain edge and the latter being ComboEdge. In both cases the M2 blades are BT2 coated since this steel type has a higher carbon content.
Just to complete the coding system discussion, we offer a "CG" (chisel-ground) blade in the Model 910.