progress is pretty much synonymous with that pool, and personally I think it's better as a tag than a pool given that it's representing something objectively present in the image. The aim of this one is largely to show the improvement in an artist's skill over time.
before and after is for stuff like before and after a change in, for instance, a character's appearance or personality. These are usually drawn by the artist at the same time, and even when they aren't the aim is to show the change in what is being drawn, rather than in the artist's ability.
There's some degree of obscurity on the boundaries, and a definite overlap between the last two, but that's the case with quite a few tags, and I do think they all deserve their own distinct tags. Of course, they all have some examples of mistagging as well.