It's not referring to any actions in particular.
The thing about writing RPGs like Starfinder is that there's a general expectation that you're going to publish more books. That means in the books you write early on, especially the core rulebook, and super especially the basic rules that apply to everyone, you want to be kind of careful about how the rules you write may combine with material that will come out later that you haven't even imagined yet. The writers aren't so much thinking, "We need to make sure players don't combine Total Defense with X," as they're thinking, "We should probably keep players from combining Total Defense with other stuff unless we come up with something later that we do want to combine with it, in which case we can just say that thing is allowed to combine with Total Defense despite Total Defense not combining with most things."
This philosophy is called "exception-based design": create a set of broad rules covering as much as possible, and then when things need to break the rules add specific text to the thing saying, "This is allowed to break such-and-such rule." It's a fairly common approach in RPGs, especially in D&D and its offshoots (of which Starfinder is one).
At the moment there's maybe only one or two actions covered by that clause in Total Defense (the level 14 solarian revelation Gravity Shield was the only one I found in a quick search). Total Defense probably wasn't written with those specific exceptions in mind, however, so much as it was written with "any possible action we might publish in the future" in mind.