Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Reduction Strength
POLYREDUCTIONOBJECT_STRENGTH
Parameter: Mesh Quality Factor
POLYREDUCTIONOBJECT_MESHFACTOR
Parameter: Co-Planar Optimization
POLYREDUCTIONOBJECT_COPLANAR
Parameter: Bounday Curve Preservation
POLYREDUCTIONOBJECT_BOUNDARY
Parameter: Polygon Quality Preservation
POLYREDUCTIONOBJECT_QUALITY