It's in a sqlite database. Unison doesn't prescribe how those are passed around, but since everything is content addressed there are never any name collisions. You can always merge two of them into one without having to resolve conflicts and without breaking anything. Doing so just expands your codebase to include more code than you're currently calling.

The way I intend to use it is that users periodically assemble the codebases from all of the developers that they care about and merge them into a single codebase. This can be very latency tolerant (like, sneak-a-thumb-drive-across-the-border levels of latency tolerant) because it's not like you're waiting around for transport every time you want to do something, just every time you want to upgrade.

Then, when you decide to use the new version of something (the developer would communicate a new hash to you), you just start calling the new hash and now you're using the new software. If something goes wrong, you just go back to calling the old hash. No implicit trust (e.g. no need for SSL, DNS, package names), atomic upgrade/downgrades, no merge conflicts.

It’s a binary-encoded merkle tree