Is the meaning of "lock-free" even defined by the C++ standard?

I can't find a semantic difference between lock-based and lock-free atomics. So far as I can tell, the difference is semantically meaningless as far as the language is concerned, since the language doesn't provide any timing guarantees. The only guarantees I can find are memory ordering guarantees, which seem to be the same for both cases.

(How) can the lock-free-ness of atomics affect program semantics?

i.e., aside from calling is_lock_free or atomic_is_lock_free, is it possible to write a well-defined program whose behavior is actually affected by whether atomics are lock-free?
Do those functions even have a semantic meaning? Or are they just practical hacks for writing responsive programs even though the language never provides timing guarantees in the first place?