Honest question: what's the difference? Observable "precedent" has fallen flat on its face completely time and time again. People are reduced to arguing that Min Min only bends it instead of actually breaking it. And that's not getting into stuff like Byleth and Sephiroth. I could use the search function and in 15 minutes come back with a huge wall of shame of posts absolutely certain characters like those simply wouldn't happen because of rules... sorry, because of observable precedent that as it turns out never actually meant anything.
I'm not trying to demean anyone by the way. After all, I myself was very, VERY adamant that games with representation in the base game would NOT get a character and that blew up in my face spectacularly. The reason I'm so against this is nothing more than me recognizing it for what it is: an extremely flawed method for speculation. It also bothers me immensely that people try to equate speculation with merely noticing patterns and stuff, like there's nothing more to it, and that if you make fun of that you are somehow against the very purpose of this thread.
The difference is pretty simple;
"It can only happen this way" is a fan rule. Observable Precedent simply means "I'm doubting it done differently, but I won't deny it being possible".
Fan rules are used to shut down speculation. All the idea that AT's are impossible to ever upgrade to playable in the same game, or Spirits, for two good examples. Precedent is just giving a reasonable argument based upon why you doubt something. Let me note a quick example from myself; I knew Isabelle would not be an Echo. She literally lacked the same bodyshape as Villager, making it impossible. The rest of them were literally the same bodyshape(with at best, inches of differences). I won't go much into it, or brag, etc. I just observed something clear about Echoes(due to an official statement about them), but overall agreed that beyond bodyshape, it's not clear. I.E. things like weight, stats, etc. Now do note that Ken shook up speculation later on, which really made us question if anything beyond "same bodyshape" mattered. Especially with Dr. Mario in, who is not an Echo. It's an odd thing.
I've noted a lot of times why AT's and Spirits are not impossible too. Here's another good example;
"Spirits already have a role, therefore, they blatantly cannot be playable." This is an outright fan rule. Because it's not about doubt, it's about trying to shut down anything that supports them.
"Spirits were in base game, so I doubt they're in Pass 1 since Pass 1 was probably chosen with them in mind." This is observable precedent, since it also comes with the point that relates to AT's, as we never saw an AT get promoted in 4. The same logic is being used here to doubt it.
"Spirits shown in Pass 1 totally cannot be playable in Pass 2". This is still a case of a fan rule. However, most times people just mean it as an observable precedent, since it's pretty much like the above paragraph. A few will try to treat it like a fan rule, though. Others will think "wait, why was it chosen that way".
"AT's cannot be playable. They already have an in-game role." Definitely a fan rule, in the specific way it's stated here.
"AT's already have an in-game role. I doubt they'll change that for DLC." Observable precedent, again, from Smash 4. Even if it's literally "one case", that's still a precedent set that people will use to figure out how the game's roster will be chosen.
You can apply things to Mii Costumes too, Trophies in Smash 4(which note that the only Trophies that got playable were actually veterans. It's one of the more silly arguments, since Trophies don't even have a gameplay role and are just a model. Spirits may be "just a jpeg", but they do affect gameplay, making it a somewhat different situation).
I hope that clears it up. There's more examples, but that should give you the idea either way.