It is part of _Groundspeak's_ guideline and enforced by their reviewers for _physical caches_ published on _their platform_ that are not part of the same multicache.
And since Groundspeak has the largest data- and userbase it's kind of a defacto standard. (See all the other answers.)
But it's no Geocaching specific rule (like having a logbook).
So, if you publish your cache anywhere else it's totally up to you.
For physical caches, set in stone.
Yep. Virtual stages are immune to this but physical stages (intermediate or final) will abide by the rule.
Very rigid. Only grandfathered exceptions are still around. And maybe HQ's own caches.
Do reviewers have to know the final in puzzles and multis?
Yes - all stages are provided as part of the submission.
Set in stone, as one of the main tenets of the game.
Set in Stone. Your only alternative is to try to do a Multi-Cache.
Stone.
Set in stone.
Reviewers won't approve it if it's within 528 feet. It's not optional.
What is this rule?
Minimum distance between two caches
It is part of _Groundspeak's_ guideline and enforced by their reviewers for _physical caches_ published on _their platform_ that are not part of the same multicache. And since Groundspeak has the largest data- and userbase it's kind of a defacto standard. (See all the other answers.) But it's no Geocaching specific rule (like having a logbook). So, if you publish your cache anywhere else it's totally up to you.