I’d also appreciate such a documentation. What @GeorgeScott calls siteId is also often present in Forge event payloads as cloudId, just without the ari:cloud:jira::site/ prefix.
This seems to be a stable identifier not only when uninstalling/installing apps, but also when different apps get installed onto a tenant - and thus, making it easier for cross-app communication.
The cloudId is already present in connect (on forge) install events, Forge install events, Forge invocation tokens, other Forge events - please provide the guarantee that the cloudId/siteId will stay.