Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Strength
MDATA_FLATTEN_WEIGHTParameter: Mode
MDATA_FLATTEN_METHODParameter: Axis
MDATA_FLATTEN_AXISParameter: Position
MDATA_FLATTEN_PLANEPOSITIONParameter: Individual Islands
MDATA_FLATTEN_INDIVIDUALISLANDSParameter: Outline Only
MDATA_FLATTEN_OUTLINEONLY