The capabilities of the Interface classifier are made available when you code a Interface type, which describe the functionality that you want to see coded in an EGL Class or Handler type.
After the Interface type is complete, you or others can code the Class or Handler type, which is said to implement the interface. The meaning of that phrase is that the Class or Handler type contains every function described in the Interface type. The Interface type provides a kind of contract that the other type must fulfill.
Using an Interface type as a design tool helps developers and business analysts think clearly about what operations are needed before other development begins.