You can create the type with:
type mixedType = A | B | Sliding of float32
then you convert the flags/float values to a value of this type with:
let getMixed af bf sliding = match (af, bf, sliding) with
| (true, _, _) -> A
| (false, true, _) -> B
| (false, false, s) -> if s >= 0.0f && s <= 1.0f then Sliding(s) else failwith "Invalid sliding value"