The documentation says

> Package atomic provides low-level atomic memory primitives useful for implementing synchronization algorithms.

> These functions require great care to be used correctly. Except for special, low-level applications, synchronization is better done with channels or the facilities of the sync package. Share memory by communicating; don't communicate by sharing memory.

There was a long mailing list discussion discussing use cases and certain desired behaviours that the people on the list agreed upon. In the end they chose not to make those behaviours explicit and leave it somewhat open-ended and instead put in the phrase "great care" to cover it off, rather than define it.