There's definitely a gray-line there, not just about how unilateral it is (which is what you are saying), but also about norms and expectations.
"Fill out this HR form or leave" is expected for the first day of work.
Similarly "Not playing WoW for 12 hours a day" is probably expected in a committed adult relationship (unless of course this was a norm during courtship).
RTO might well be a norm for people not hired remote. But forcing those hired as remote workers to relocate seems outside of the norms. It's the difference between "Do your job, or your fired" and "Switch to this different job, or your fired."
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I think there is a third leg to making something an ultimatum, and it's about the degree to which the move is (or is perceived to be) about imposing ones will on another.
"I don't want to be married to someone who plays WoW for 12 hours a day," is a different tone from "Quit WoW, or I'm leaving you," and it's natural for someone to respond differently to those two statements.
I'm looking through the responses, and this makes the most sense.
The difference between saying that '12 hours is too much', depends on how it is said and what is open to negotiation to keep both sides happy. The thing is that 12 hours may be too much, but 4 hours, for example, might keep both sides happy. Also saying '12 hours is too much' is also an opening gambit that states what the issue is, and opens the door to how two people might come to an agreement.
Ultimatums tend to come in the form of offering a binary choice, one of which is full acceptance of a proposal made by one person. Ultimatums don't come with a 'or we can negotiate something that works for us' addendum, they are usually all or nothing.
While the first type of communication / boundary may well lead to a divorce if it can't be resolved, it isn't necessarily presented as an ultimatum.
An ultimatum is a my way or the highway choice. "Submit or suffer".
Negotiating how much time you invest into the relationship isn't an ultimatum. "Quit WoW entirely or I'm getting a divorce" is.