Do you mean a reference selector, where you select an object to store in an association of type reference? Or a drop-down widget where you make a selection from an enumeration?
In the reference selector you can show any attribute of the object you are going to select and the value that you select is an object, not a simple value.
The drop-down shows the value of the enumeration. Each enumeration value has name (used internally) and a caption (used in the user interface). The caption can be an arbitrary text, like "Central European Time (GMT + 1)".