…(which is: identifying a specific reboot cycle universally and uniquely), the invocation ID does for an individual unit (which is: identifying a specific start cycle universally and uniquely).

Because of the conceptual similarity between boot IDs and unit invocation IDs, in v257 we decided to expose them in a similar way as boot IDs in journalctl.

Or in other words: for boot IDs you can use journalctl --list-boots/--boot=/-b to enumerate or filter, or focus on most recent boot.