Roles

No object exists in isolation. It is always part of a bigger machine. To fit in, an object has a specific purpose—a role it plays within a given context. Objects that play the same role can be interchanged. For example, there are several providers that can deliver letters and packages: DHL, FedEx, UPS, Post, Airborne. They all have the same purpose, if not the same way of carrying out their business. You choose from among them according to the requirements that you have for delivery. Is it one-day, book rate, valuable, heavy, flammable? You pick among the mail carriers that meet your requirements.

A role is a set of responsibilities that can be used interchangeably.