var://system/variable Addresses a global variable that is available in all contexts. System variables persist beyond the scope of request-response processing and can be read by other objects on the appliance. If the content of a variable needs to be read or set outside the scope of request-response processing, use a system variable.
The words "other objects on the appliance" imply it is visible across all domains, although all other variable types described in the section are scoped to a domain.
So in other words, if I want a system variable to be domain specific, do I need to name it var://system/mydomain/variable or is var://system/variable good enough?