I would like to configure a not so common start sequence with systemd: Service A is required by service B only while service B is starting. Once Service B started, Service A can be stopped and Service B can go on running.
If Service B tries to start, and Service A is stopped, then one of these must happen: Service A must start or not. If not, Service B must not start.
So, Service A is required by Service B only at start time of B. Otherwise, Service A is irrelevant.
I have checked the system.unit options (requires, requisites, wants, before, after, etc), and I cannot find the right mix to achieve such behaviour.