Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Sphere Radius
ATOMARRAYOBJECT_SRADParameter: Cylinder Radius
ATOMARRAYOBJECT_CRADParameter: Subdivisions
ATOMARRAYOBJECT_SUBParameter: Single Elements
ATOMARRAYOBJECT_SINGLE