Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Particle Object
FIELDLAYER_PARTICLE_OBJECTParameter: Radius
FIELDLAYER_PARTICLE_DISTANCEParameter: Clip to Shape
FIELDLAYER_PARTICLE_CLIPParameter: Color Alpha
FIELDLAYER_PARTICLE_COLOR_ALPHA