Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Position
OUT_PART_POSITIONParameter: Velocity
OUT_PART_VELOCITYParameter: Age
OUT_PART_AGEParameter: Life
OUT_PART_LIFEParameter: Size
OUT_PART_SIZEParameter: Scale
OUT_PART_SCALEParameter: Mass
OUT_PART_MASSParameter: Alignment
OUT_PART_ALIGNMENTParameter: DT Factor
OUT_PART_DTFACParameter: Random Seed
OUT_PART_RANDOMSEEDParameter: Color
OUT_PART_COLOR