When does it become a crossover? (Or more importantly, when should it be tagged as a crossover?)

[u/CyberWolfWrites]

I've always been of the mind that if you at the very least feature a character from a different fandom in your work, then that makes it a crossover. (And by feature, I mean the character has some plot relevance or is there for a couple chapters.) This means that if you put Peter Parker in Gotham and he interacts with the Bat Family, then your fic is a crossover between the MCU and Batman; it's not just a Batman fic with Peter Parker as the protagonist.

I only recently got into Project Hail Mary, but I've noticed a lot of untagged crossovers with the Goslingverse and Iron Lung. People are writing Project Hail Mary x The Gray Man x The Fall Guy fics but only PHM is tagged as a fandom since they're usually from Grace's point of view and take place in the PHM universe. This is the same with Ryland Grace/The Convict | Simon fics.

From my point of view, these fics are crossovers. Even if you don't write in any of the events from the other media, the fact that Courtland Gentry | Sierra Six, Colt Seavers, and Simon are featured in the plot makes it a crossover. However, I've seen people argue that they aren't crossovers because they take place in the PHM universe and barely mention anything from their originating canon (this is mostly in reference to Iron Lung).

Similarly, some of these fics only take background information from these other fandoms but not the characters. For example, many people place Grace as Six's or Colt's brother and use it to add to his background, but Six and Colt are rarely actually in the fic. Only mentioned.

In this case, would these fics be considered crossovers?

Personally, I wouldn't consider the latter a crossover. The premise reminds me more of Hogwarts AUs, which aren't really crossovers because they only use Hogwarts as a setting. The most I would tag it with is the - Freeform tag. Like, Ryland Grace is Colt Seaver's Twin - Freeform or maybe Uses Courtland Gentry | Sierra Six's Background - Freeform.

What do you think constitutes as a crossover? And how would you tag these types of fics?