What makes something Off Broadway?

Ok, I have a question.

My understanding is that Off Broadway means that your show is performing in a theatre in Manhattan that's 99 seats or more (but not necessarily an Equity production). I don't want to name names, but I've been seeing a few shows offered on TodayTix that are listed at Off-Broadway theatres (there is one particular one that has been getting a lot of press that I know for a fact is in a theatre with around 60 seats) that I know for a fact are under 99 seats and not technically Off Broadway theatres.

But then again, I've always wondered, would a large scale immersive show like SLEEP NO MORE constitute as Off Broadway? Or does it live in a weird nebulous realm b/w Off Broadway and Off Off?

Similary, could you take a venue that is traditionally Off-Off in Manhattan and add more seats into it and it would become an Off Broadway theatre?

I also couldn't find eligibility requirements for the OBIEs or the Off Broadway Alliance's website, which I thought would be the proper place to look. I guess the root of my question is who makes the call if something is Off Broadway or not? Could a show in an Off Off venue but is completely Equity and has a star in the role or something elevates its status as an Off Broadway show, too? Also, is an Off Broadway house automatically mean it becomes an Equity house? If so, what about all the non union shows that are in theatres like Actors Temple or the recently departed St. Lukes (RIP).

The whole "rule" about the number of seats determining Broadway vs. Off-Broadway vs. Off-off-Broadway, is only partially based in fact. 

If I understand it correctly, the "official" Off-Broadway theatres are mostly between 100-500 seats, just as all official Broadway theatres are 500+ seats, but the reverse is not necessarily true. Theatres can be 500+ seats and not be Broadway houses, just as theatres between 100-500 may or may not be considered "Off-broadway" under official designations.

Then you have weird loopholes like the Delacorte, which is officially considered Off-broadway despite being over 500 seats, because it's produced by the most prominent "Off-Broadway" company. And I believe their Shiva theatre is only 99 seats, yet their productions are considered Off-broadway when they're part of the mainstage. And yet, over at 59e59, their productions are considered "Off-Broadway" officially, but not in their smallest space. 

I believe veronicamae is correct that there's some sort of opt-in process, just as there is for Broadway. And that's really the most relevant factor for what's officially considered "Off-Broadway." And I'm sure it also affects the terms on the union contracts, in terms of pay scale, etc. 

But then, the term also has a more colloquial usage, which is just to refer to non-Broadway productions that are somewhat high profile. St. Ann's Warehouse, BAM, and TFANA are not officially "Off-Broadway," yet they're producing some of the most high-caliber, and even high-profile work in the city. So really, the whole "official" designation of what's "Off" vs "Off-off" is largely BS as far as the cultural landscape of NYC theatre is concerned. As far as the general public is concerned, the terms are useful in describing different kinds of theatre companies, based on their resources and notoriety, but paying attention to the nitty-gritty industry designations can be misleading and unhelpful. 

Correct me if I'm wrong, but I believe the Drama Desks don't descriminate between "Off" and "Off-Off." I believe any production in NYC is eligible if they submit and invite voters. Whereas, I believe the Obies and Lortel awards are more restrictive.