It's targeted at distributed systems, but it can be used to model any system over time. I've used it for distributed systems, but also for embedded systems with a peculiar piece of hardware that (seemed, and we found was) to be misbehaving. I modeled the hardware and its spec in TLA+, then made changes to the behavior description to see if it broke any expected invariants (it did, in precisely the way we saw with the real hardware). The TLA+ model also helped me develop better reproducible test cases for that hardware compared to what we were doing before.