This point is important, so I'll repeat it:
A snapshot is identified by the SHA1 sum of the message file that uniquely identifies it (and its parent).
These snapshots can be created and moved around between computers without losing their identity or where they belong in the history tree of a project.
Finally, snapshots can be shared or kept private as you see fit. If you have some experimental snapshots that you want to keep to yourself, you can do so quite easily: Just don’t make them available to Zoe!