Top-level collections

A top-level collection is a grouping of functionality that targets a particular software consumer (or type of consumer) of the functionality. For example, a top-level collection can be described as keyboard, mouse, consumer control, sensor, display, and so on. In the HID spec, these top-level collections are also referred to as application collections. The HID device describes the purpose of each top-level collection, allowing the consumers of HID functionality to identify top-level collections in which they're interested. In Windows, the HID device setup class (HIDClass) generates a unique physical device object (PDO) for each top-level collection described by the report descriptor. Microsoft defines a top-level collection as a HID collection that isn't nested within another collection. An unnested collection is always a top-level collection, regardless of its HID type. In particular, a top-level collection doesn't have to be an application collection, as defined by the USB HID Standard.

A report descriptor can include more than one top-level collection. The HID class driver enumerates the top-level collections of an input device and creates a physical device object (PDO) for each top-level collection. User-mode applications or kernel-mode drivers can access a top-level collection by opening its PDO and using the HIDClass support routines and the HID class driver IOCTLs.

The following list describes the internal structure and capability of a top-level collection:

• A HIDP_CAPS structure summarizes a top-level collection's capability.
• Link collections describe the organization of the nested collections contained within a top-level collection.
• Button capability arrays and value capability arrays describe the capability of the controls supported by the top-level collection.