windows-server-2003/drivers/wdm/input/hidclass/schema.txt

For each HID minidriver in the system, the HID class driver keeps a 
HIDCLASS_DRIVER_EXTENSION, like so:

HID.SYS -+--> HIDCLASS_DRIVER_EXTENSION
	 |
	 +--> HIDCLASS_DRIVER_EXTENSION
	 |
	 .
	 .
	 .

For each device object associated with a minidriver, the class 
driver keeps a HIDCLASS_DEVICE_EXTENSION, like so:

HIDCLASS_DRIVER_EXTENSION -+--> HIDCLASS_DEVICE_EXTENSION
			   |
			   +--> HIDCLASS_DEVICE_EXTENSION
			   |
			   .
			   .
			   .

For each collection associated with a device object, the class driver keeps 
a HIDCLASS_COLLECTION structure:

HIDCLASS_DEVICE_EXTENSION -+--> HIDCLASS_COLLECTION
			   |
			   +--> HIDCLASS_COLLECTION
			   |
			   .
			   .
			   .

For each file object associated with a collection, the class 
driver keeps a HIDCLASS_FILE_EXTENSION structure:

HIDCLASS_COLLECTION -+--> HIDCLASS_FILE_EXTENSION
		     |
		     +--> HIDCLASS_FILE_EXTENSION
		     |
		     .
		     .
		     .

Finally, the class driver keeps a list of IRPs that are pending against 
each file object:

HIDCLASS_FILE_EXTENSION -+--> IRP
			 |
			 +--> IRP
			 |
			 .
			 .
			 .