What is exactly "DIRECTION_UNKNOWN" for Call.Details class?

Question

Background

I'm investigating how CallScreeningService works (sample repository here, easier one here, if anyone wishes to check out).

The problem

I've noticed that for callDirection there is incoming, outgoing, and "DIRECTION_UNKNOWN" :

Indicates that the call is neither and incoming nor an outgoing call. This can be the case for calls reported directly by a ConnectionService in special cases such as call handovers.

I don't understand the meaning of this.

What I've tried

I tried to see if I can cause normal phone calls to trigger it, but of course it's not as such.

The question

What kind of special case is it? What is "call handovers" ?