Well, the Constant setting is pretty much as it sounds. You “paint” a spot or area and it remains for the remainder of the clip. You can then delete keyframes if you wish it to end at a certain time. I had to use this technique to clean up an interview (of Christopher Lee) where the original BetaSP tape had oxide flaking off leaving several frames of while flecks throughout the entire clip. I could advance or retard from those frames, clone from a different time that was clean, then turn off the paint effect when the glitch ended.
I don’t know if newer CC versions of AE have the behavior of the PS tool or not. I’m still happily using CS6 and not having to suffer broken plug-ins when an SDK isn’t release in a timely manner.