Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Minimum
FIELDLAYER_CLAMP_MIN_VALUEParameter: Clamp Min
FIELDLAYER_CLAMP_MIN_ENABLEDParameter: Maximum
FIELDLAYER_CLAMP_MAX_VALUEParameter: Clamp Max
FIELDLAYER_CLAMP_MAX_ENABLED