A service-element is either an initiator or a responder,
and contains zero or more functional-units.
For example:
An ACSE object (aACSE) is composed of a role
object and a functional-units object.
A VT object ( a m contains no functional-units,
and therefore only contains a role object.
Thus each instance of a concrete subclass of the
abstract-service-element is a composite object which
is always composed of the following items (Figure 8):
an instance of a concrete subclass of abstract-role,
at most one instance of a concrete subclass of
abstract-functional-units.