SA-21 was initially S-300PM-3 before it was renamed S-400 (hype bonus). PM-1/2 are SA-20, PS and PT are SA-10.
SA-20 is midway between SA-21 & SA-10. Main improvements of SA-21 are electronics and fire-control wise (harder-faster-better-stronger, basically) and the employment of an increased variety of missile types to be able to engage threats the most efficiently possible way within all the battery’s area of responsibility.