The definition of the AUDIT_DETAILS.system_id is not 100% clear: "Identity of the system where the change was committed. Ideally this is a machine- and human-processable identifier, but it may not be".
Some implementers use the system_id of the commit service/server (a technical id), others use this as a system id more at a logical / organizational level (for multi-tenant systems maybe the same commit service / server is used as backend of different systems and each of those systems client + backend have a different system id).
In a distributed system, maybe a set of clients + backend servers have only one system id, like in a nation-wide or regional EHR system.
Also about the id itself, Thomas mentioned that was thought to be something like a URI, but a lot of implementations are using UIDs.
So, the definition leads to different interpretations and identification schemas. It might be good to clarify the specification and define a specific id schema like using just URIs or UIDs or something else. I would prefer to use random UUIDs because those can be generated in a distributed way and URIs might need some level of coordination / centralization.