UML25 — Location: 18.1.1 Summary P. 685 - A Subject may only refer to a single system

current text: “A UseCase’s subject represents the system or systems under consideration to which the UseCase applies.”

[AC] This addition wasn't in the previous UML version, 11-08-06, where the description was “The subject is the system under consideration to which the use cases apply.”

I consider the plural “or systems” ambiguous, because it lets the reader think that the subject may refer to more than a single system.
If we want to say that the subject may be a very complex one, such as a system of systems (and this may be indeed useful in my opinion) we should state this point explicitly.

So I suggest to go back to the previous formulation, or to state more explicitly that the subject may refer to an arbitrarily complex system

Reword as suggested.