Huh. Sounds like this is something we should be asking staff about. If it does something, we should probably know what. If it doesn't, then they really should have removed it.
Edit:
Okay. I did some more digging. It appears this is part of the core MediaWiki. I am guessing it was just one of the things changed for legacy wikis when Fandom created its fork; or it was added after the fork. Either way, since it is part of the core MediaWiki, that explains why we are seeing it on UCP wikis.
It is briefly mentioned in the documentation on importing but it is literally just one sentence that says nothing more than "its there, use it". According to discussions on the meta site, it is there to ensure that proper attribution is given to the source wiki. However, I either didn't dig far enough or it just isn't stated how it does that. So those details are still something that might be of interest to ask staff or users like KockaAdmiralac who are closer to the core code then the rest of us.
Based on the discussions on the meta wiki, I believe the general idea is this. The destination wiki specifies an interwiki prefix for the source wiki in the interwiki map. You then enter that prefix when importing the pages and, somewhere (I have no idea where), that information is used to auto-generate links to the original. Thus attribution is assured. Obviously this presents an issue for Fandom wikis where admins can import pages from wherever but can't even (easily) view the interwiki map.
The workaround given on the meta wiki is the same Sophiedp mentioned; just make up a prefix. However, the discussions on the meta wiki also mentioned that it is best to use a different prefix for each source wiki. This suggestion appears to be consistent with my assumption about how it is intended to be used. If this is the case, then one would not want to just make up a prefix but also make sure it isn't one that actually is in use or has a reasonable chance of being added.
Again, admins can't easily check the interwiki map on Fandom wikis, so verifying that the prefix is not already in use is yet another issue. Given the apparent purpose of the prefix field, I think it makes sense to use either the Fandom-to-Fandom interwiki linking syntax or the source wiki's domain (i.e. the Fandom-to-Fandom interwiki syntax but with the "w:c:" removed from the front). But that is just my opinion.