An example of an Ultimatum (If you don't do this, I'll do this):

Someone needy or a "beta" might tell a girl that he can find other girls that can value his time and so he's not going to waste time with her. But by telling her that, he's showing that he cares too much and is creating an ultimatum of "make time for me OR I leave", when instead, he should have just hard nexted her.

On the other hand, someone "alpha" with outcome independence and options would straight up just ghost a chick who is not giving back any investment because he knows he can't change a chick and that she's not worth his time or even the effort of an explanation. He wouldn't be salty either because the saltiness guys have likely comes from thinking they lost their "better option" from a place of lacking options.

An example of a Boundary (I won't accept you doing this to me):

If a girl tries to be disrespectful and take things and expects you to just let them (ex. eating food from your plate, calling you a retard, or anything else like that) someone "alpha" will either reinforce the boundary in a neutral or teasing way, or just not fuck with the chick if they see red flags. Like a chick who tries to shit talk you in front of others, even if you enforce a boundary, she's still a shit person and not worth an LTR, so it's on you what you want to do with that.

A "beta" will either let it happen or get SUPER offended about it. Like imagine a "beta" reads some RP theory for the first time and a chick tries to eat something from his plate, and then he gets salty af and says "if you do that again, I'm leaving the table", which seems like a boundary but is likely an ultimatum because they're getting super offended for something not worth getting that offended about. Or, on the other hand, he might just let her and lets other things slide while smiling and trying to be nice.

The Issue:

It might seem like there's a clear difference between the two, but in practice, that's not always the case. A boundary can come off as an ultimatum in some cases, and this can make you come off as easily offended. For example, if someone tries to attack you in a group, let's say you only agree and amplify. Well, if you don't talk with them and set a boundary, they'll keep doing it because you didn't set a limit. But at the same time, setting a boundary in a situation like that can make you come off as overly sensitive. Like where is the limit of boundaries vs. ultimatums and being sensitive?