The way the "Any" matching works on the AO3 back-end is that if you offer Any, you can be matched to anyone requesting that fandom, no matter what characters they are requesting. If you request any, you can be matched to anyone offering the fandom, no matter what characters they are offering. Most exchanges that allow Any still require authors to write about characters from the tagset.

We're doing it a little differently. When we run matching, the code will work the same way to match people that it does for other exchanges. And if you offer Any, you will still have to write a character your recip requested, just like in other exchanges. But if you request Any, your author can write any character appearing in that fandom (which for an umbrella fandom could include literally any character from any of the sub-fandoms). So we wanted to point it out in the nominations post so that people can plan their noms accordingly.

(As for why we're doing it this way: basically, we think it could be fun to interpret Any this way for requests, so we're trying it out for this round. If it ends up not being fun, we'll change what we're doing with it for the next round, or remove Any altogether if it's too much of a headache.)

I hope this helps!