How to use USB Feature Report with PlugableUSB

Dear Community,

I want to develop an USB HID interface that can also handle setup requests from the host. All the examples I can find do only contain INPUT and OUTPUT elements in the report descriptor. For the interface I want to implement I need the REPORT descriptor.

Simplified report descriptor is like this:

COLLECTION REPORT_ID (1) USAGE (ConfigValue1) FEATURE (Cnst,Var,Abs)

REPORT_ID (2) USAGE (ConfigValue2) FEATURE (Cnst,Var,Abs)

REPORT_ID (3) USAGE (Value3) FEATURE (Cnst,Var,Abs,Vol)

USAGE (Value3) INPUT (Cnst,Var,Abs,Vol) END_COLLECTION

Problem is, I nowhere found a hint on how to link these ConfigValues to variables. As I understood there should also be a callback that is called whenever the host requests a ConfigValue. Can anyone help here?