Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Clear
MGWEIGHTTAG_CLEARParameter: Invert
MGWEIGHTTAG_INVERTParameter: Use Fields
MGWEIGHTTAG_USEFIELDSParameter: Use Deformed Points
MGWEIGHTTAG_FIELDDEFORMParameter: Fields
MGWEIGHTTAG_FIELDS