Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Position
IN_PART_POSITION
Parameter: Velocity
IN_PART_VELOCITY
Parameter: Life
IN_PART_LIFE
Parameter: Age
IN_PART_AGE
Parameter: Size
IN_PART_SIZE
Parameter: Scale
IN_PART_SCALE
Parameter: Mass
IN_PART_MASS
Parameter: Alignment
IN_PART_ALIGNMENT
Parameter: DT Factor
IN_PART_DTFAC
Parameter: Random Seed
IN_PART_RANDOMSEED
Parameter: On
IN_PART_ON
Parameter: Color
IN_PART_COLOR