Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Reference
GV_HAIR_COLLISION_OBJECTParameter: Collision Index
GV_HAIR_COLLISION_INPUT_NUMParameter: Count
GV_HAIR_COLLISION_OUTPUT_COUNTParameter: Guide
GV_HAIR_COLLISION_OUTPUT_GIDParameter: Segment
GV_HAIR_COLLISION_OUTPUT_SIDParameter: Segment Delta
GV_HAIR_COLLISION_OUTPUT_SEG_DELTAParameter: Polygon Index
GV_HAIR_COLLISION_OUTPUT_POLYINDEXParameter: Hit Point
GV_HAIR_COLLISION_OUTPUT_HITPOINTParameter: Velocity In
GV_HAIR_COLLISION_OUTPUT_VELOCITY_INParameter: Velocity Out
GV_HAIR_COLLISION_OUTPUT_VELOCITY_OUTParameter: Frame Delta
GV_HAIR_COLLISION_OUTPUT_FRAME_DELTA