A device may have more than one interface. For example one interface may use bulk transfers while an alternate interface may use isochronous transfers. Each interface has a collection of endpoints that each interface uses for device control. Endpoints may not be shared between interface; endpoint 0 is the exception as each interface uses EP0 for device control. The Set Iface button on the EZ-USB Control Panel is used to select an alternate interface to use when the device has more than one interface available.
Chapter 2 of the EZ-USB Technical Reference Manual (TRM), beginning on page 47 / 402, details Configurations, Interfaces and Alternate Settings. The EZ-USB FX2 TRM can be found on our website www.cypress.com/?rID=38232.