Only one configuration can be active at once. Each configuration has multiple interfaces. Each interface offers a logical function of the device. They are all active at once. Each interface has an alternate setting.
Alternate settings select how much bandwidth a device uses, programming interface, etc.
Only one alternate setting can be active at a time per interface. Each alternate setting has 1 or more endpoints. To make things more complicated, the entire device has one control endpoint, shared among every interface. This control interface is required to configure generic device parameters as well as parameters specific to each interface. Since each interface is a logical function of the device, separate permissions are required.