The Device Class Definition for HID 1.11 is intended to supplement the USB Specification and provide HID manufacturers with the information necessary to build USB-compatible devices. It also specifies how the HID class driver should extract data from USB devices. The primary and underlying goals of the HID class definition are t

be as compact as possible to save device data space
allow the software application to skip unknown information
be extensible and robust
support nesting and collections
be self-describing to allow generic software applications
HID Usage Tables
The HID Usage Tables 1.12 document defines constants that can be interpreted by an application to identify the purpose and meaning of a data field in a HID report.

Usages are also used to define the meaning of groups of related data items. This is accomplished by the hierarchical assignment of usage information to collections.

Usages identify the purpose of a collection and the items it contains. Each Input, Output, Feature, and/or Collection data item within a Collection item can be assigned a purpose with its own usage item. Usages assigned to a collection apply to the items within the collection.

The HID Usage Tables document contains extensions to the tables defined in the USB Device Class Definition for Human Interface Devices. All usages pages, except the Keyboard table, are replicated in the Usage Table document. The Usage Table document identifies the extensions to the Keyboard usage table.

Note: For keyboards, look at the usage table sections in both the HID Specification and the HID Usage Table document!