It does not refer to specific actions.
Writing RPGs like Starfinder is generally expected to release more books. That means in the books you write early, in particular the core rule and Super Above all, the basic rules that apply to everyone should be careful about how the rules you write can be combined with material that will come out later and that you can not even imagine. The authors do not think so much: "We need to make sure that players do not combine Total Defense with X, because they think," We should probably stop players from combining Total Defense with other stuff unless we have something else later than us do I want to combine with him. In this case, we can simply say that total defense can be combined, even though Total Defense does not connect to most things. "
This philosophy is referred to as an "exception-based design": create a set of general rules that cover as much as possible, and then add a specific text to the matter if necessary. such rule. "It's a fairly common approach in RPGs, especially in D & D and its offshoots (including Starfinder).
At the moment, there may be only one or two actions covered by this clause in Total Defense (the Level 14 Solarian revelation, Gravity Shield, was the only one I found in a quick search). Total Defense was probably not written with these specific exceptions, but as if it were meant to be "possible actions that we could post in the future".