Freehub bodies
The freehub body is the splined part your cassette mounts on — it must match your drivetrain. Open one to see which trainers offer it.
Quick answer
Shimano/SRAM 8–11-speed use HG; SRAM 1×12 MTB uses XD; SRAM road uses XDR; Shimano 12-speed MTB uses Micro Spline; Campagnolo uses its own. Direct-drive trainers ship with HG and sell the others separately.
Shimano / SRAM
Shimano/SRAM HG (HyperGlide)
8/9/10/11-speed · min 11T
ShimanoShimano HG 11-speed road
8/9/10/11-speed · min 11T
SRAMSRAM XD Driver Body
11/12-speed · min 10T
SRAMSRAM XDR Driver Body
11/12/13-speed · min 10T
ShimanoShimano Micro Spline
12-speed · min 10T
CampagnoloCampagnolo
9/10/11/12-speed · min 11T
CampagnoloCampagnolo N3W
12/13-speed · min 9T
ZwiftZwift Cog (virtual shifting)
8/9/10/11/12-speed · min 14T