It's literally making an In-Universe exchange (as in from the perspective of characters; if it's an exchange that you could conceivably use as an actual exchange and it seems completely normal, then that won't be accepted since that's not really a gift thing). Our rules for that are that it counts for "Wordcount for everything written and I'll also need the password to fake exchange account so it can be revealed at the same time if anything was actually posted to it."

To use one of my fandoms as an example, two of the characters are canonically into a fandom called Proud Immortal Demon Way (which does not actually exist, it's just a book referenced within my fandom). If I requested "Shen Yuan & Shang Qinghua" and "Medium Opt-In: AO3 Based Fanfic Exchange", someone could create a fake fandom exchange for Proud Immortal Demon Way, posting as Shen Yuan and Shang Qinghua and writing up the rules, gifts, tagset, ect from their perspective.

Another example would be Jedi RPF from the perspective of people who live in the Star Wars Galaxy, if someone requested that tag with characters that would work for that. Or maybe someone wants Kylo Ren to run a Darth Vader appreciation exchange.

(I'm going to fix that to specify in-universe really quick before someone requests/offers it)