Fig. 3.3(a) uses the idea of a card file and individual cards within it as being analogous to an entity type and an entity respectively. In Fig. 3.3(b) the set - element model is used to show the same thing, and in Fig.3.3(c) the entity-relationship model for the same situation is shown. These are three different models of the same phenomenon. Notice that the entity-relationship model version does not explicitly show individual entities. You are meant to know that 'within' the entity type CUSTOMER there are lots of customer entities