I didn't see any mention of this on the wiki pages, so I figured I'd ask here: If a series of posts is in a pool, do we apply TWYS to each individual image, or to the pool entirety? I'm looking through some older images and noticed it came up on post #420148, without any admin ruling.

That's my personal policy too. I do believe some common sense is allowed.

Aat the end of the day the official answer is that TWYS always applies to each individual image in isolation; whether they are in a pool or not is actually irrelevant. With gender-related tags this is doubly true.

In practice, posts in pools tend to get tagged a bit more loosely than individual posts. This is partially due to the effort needed to tag them, and partially due to certain progression-related tags ending up on more images than technically required (e.g., vore, growth/inflation, transformation, etc.).