I'm not sure how all of this really shakes out formally, but one of the cool things about using a SAT solver for dependency resolution is that it is guaranteed to be complete: if a solution exists to the dependency resolution problem presented by your package management operation, zypper/libsolv is guaranteed to find it.

But sometimes, a weird thing may happen, and no such solution will exist. In a case like that, the best zypper can do is offer to amend your request in various (hopefully) minimal ways, and ask if you agree that any of those outcomes are 'good enough'. IME, zypper is pretty good at this. But unfortunately, answering those questions may require you to have a clearer picture of your system's state and your goals than many users/administrators do.

The safest thing to do when presented with those prompts is, of course, to abort. But snapper definitely makes messing around and trying your options less punishing than it might be!