Curved-tooth profiles look interchangeable on the shelf, but each has a clear job.
HTD (high torque drive) is the workhorse — it transfers more torque than a trapezoidal tooth at the same pitch, and runs quieter. Default pick for new drives above 1 kW.
RPP (RPP profile / STD) uses a modified involute tooth shape that has zero radial play. Pick this when the drive is a positioning axis on a machine tool or robot, where backlash would be unacceptable.
STD is the older "STD" name for RPP. Pick STD only when you are matching an existing drive that uses that nomenclature.