So I was looking at dresses on F21, and I noticed that some were XL vs 1X? What's the difference?

Here's a link to Forever 21's plus size chart. If you click on size chart in most of the plus size website fashion, it takes you to regular sizes, not plus.

1x is a size bigger than XL.

XXS, XS, S, M, L, XL, 1X, 2X, 3X...so on. 1x is bigger. 1x is the beginning size of plus size.