Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Reduction Strength
POLYREDUCTIONOBJECT_STRENGTHParameter: Mesh Quality Factor
POLYREDUCTIONOBJECT_MESHFACTORParameter: Co-Planar Optimization
POLYREDUCTIONOBJECT_COPLANARParameter: Bounday Curve Preservation
POLYREDUCTIONOBJECT_BOUNDARYParameter: Polygon Quality Preservation
POLYREDUCTIONOBJECT_QUALITY