USB Library
USB_HOST_APP_EVENT_HANDLER Macro
Syntax
#define USB_HOST_APP_EVENT_HANDLER(a,e,d,s) ((e==EVENT_OVERRIDE_CLIENT_DRIVER_SELECTION)?false:true)
Description

If the application does not provide an event handler, then we will assume that all events function without error.

MLA - USB Library Help Version : 2.16
http://www.microchip.com/mla