Rootless voicings drop the root (the bassist plays it) and add the 9th for colour. Type A stacks 3rd-5th-7th-9th from the bottom. Type B rearranges the same notes with the 7th on the bottom: 7th-9th-3rd-5th. Alternating A and B in a progression gives you the smoothest possible voice leading.
Type A: 3rd + 5th + 7th + 9th (ascending). Type B: 7th + 9th + 3rd + 5th (7th dropped an octave below the 3rd).
Your primary voicing when playing with a bassist. The A/B alternation system is what professional pianists use in combo settings. Essential for ii-V-I progressions — the 7th of one chord resolves stepwise to the 3rd of the next. This is the Bill Evans innovation.