|USB : USB Classes A Closer Look at HID Class|
A USB HID Report is the actual data that the device sends or receives.
By contrast, a Report Descriptor is a table of numbers that describes the data. A Report Descriptor only gets sent to the PC during the Enumeration process. The Report Descriptor tells the PC what kinds of Reports the device will send (i.e. whether any Input, Output, or Feature Reports are present). It also tells the PC all about the data items that are present in each type of Report.
When we say that it tells the PC all about the data items, we really mean ALL about the data items. The Report Descriptor describes each data item in exhaustive detail, telling the PC things like:
Please note that every USB HID class device has to have at least one Report Descriptor. (Yes, some devices need more than one.)
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