Trx behaviour in Call Activity if the called process is in another engine


if there is an explanation/specification of the transactional behaviour for the following situation?

I have a process model Caller that has an activity of type Call Activity. That call activity starts a process Callee. Assume I do not set any Async attributes – neither in Caller nor in Callee. Then, in my understanding, the called instance is executed in the same transactional context (as a part of the same job) as the calling process.

But what happens if the two process models are deployed to two different process engines (each having its own job executor) that share the process and job database? The transactional context can’t be transferred from one engine to another. Is there an implicit transaction / job then? I could not find a description of this case in the docs.

Thank you in advance for clarification.