Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Gradient
XTMASK_GRADIENT
Parameter: Enable Altitude Masking
XTMASK_ALTITUDE_ENABLE
Parameter: Min Altitude
XTMASK_ALTITUDE_MIN
Parameter: Max Altitude
XTMASK_ALTITUDE_MAX
Parameter: Soften Min
XTMASK_ALTITUDE_SOFTEN_MIN
Parameter: Soften Max
XTMASK_ALTITUDE_SOFTEN_MAX
Parameter: Noise Height
XTMASK_ALTITUDE_NOISE_HEIGHT
Parameter: Scale
XTMASK_ALTITUDE_NOISE_SCALE
Parameter: Enable Slope Masking
XTMASK_SLOPE_ENABLE
Parameter: Min Slope
XTMASK_SLOPE_MIN
Parameter: Max Slope
XTMASK_SLOPE_MAX
Parameter: Soften Min
XTMASK_SLOPE_SOFTEN_MIN
Parameter: Soften Max
XTMASK_SLOPE_SOFTEN_MAX
Parameter: Use Bump
XTMASK_SLOPE_USEBUMP
Parameter: Direction
XTMASK_SLOPE_DIRECTION
Parameter: Custom Direction
XTMASK_SLOPE_DIRECTION_CUSTOM
Parameter: Alternative Blending
XTMASK_SLOPE_ALTERNATIVEBLENDING
Parameter: Hardness
XTMASK_SLOPE_ALTERNATIVEBLENDING_HARDNESS
Parameter: Use Global Coordinates
XTMASK_GLOBAL
Parameter: Invert
XTMASK_INVERT