When a system should be included as an actor in use case diagram?

I'm making a use case diagram for a new system. I'm wondering when a system should be included as an actor in use case diagram?

As stated in another answer, an actor is a system or role interacting with the system under development. You should include a system as an actor in a use case if it is outside the system you are developing, and if it directly interacts with the system you are developing.

This is important because you need to define the boundary of your system, which means its scope and interfaces. Including a system as an actor will clearly state the requirement for your system under development to provide a suitable interface for that actor system.