What are the two ways of communicating with subsystem. Explain?

There are two ways of communicating with a subsystem, either by sending stimuli to the subsystem itself to be re-directed to the proper recipient inside the subsystem, or by sending stimuli directly to the recipient inside the subsystem. In the first case, an association is defined with the subsystem itself to enable stimuli sending.

A subsystem can have generalizations to other subsystems, i.e., the public and protected elements in the contents of a subsystem as well as operations and receptions are also available to its heirs.

A subsystem may offer a set of interfaces, i.e., for each operation defined in an interface, the subsystem offering the interface must have a matching operation, either as a feature of the subsystem itself or of a specification element.