Suppose for a shopping cart application, sometimes an EJB is called by an action class or servlet. In this case, is that action class or servlet acting as the "client" ?
The client object is the servlet object. Servlet objects use many different objects to execute. In some cases, servlet objects use action objects. They get information from the action object and may also execute behavior that is coded in an action object.
So, yes both the servlet object and the action object are client objects of a Session EJB.