I write an HID driver. I found out that I need to supply the following entry in order for it to interact with the DirectInput system:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\MediaProperties\PrivateProperties\Joystick\OEM\VID_xxx&PID_xxx
I cannot find a clear and updated description of this entry.
In the Windows 95/98 days, the hardware vendor was required to describe the device capabilities through the registry. The relevant entry was OEMdate as described here.
The older versions of DirectX, it would take the definition of the hardware from this registry entry. The definition includes axis remapping (example: JOY_HWS_ZISJ2X = Z is on the J2 X axis.) and device type: Yoke, Gama pad, Race car controller and Head tracker. In addition this registry entry reports the number of buttons per device. Note that the definition is per driver rather than per device. In other words, all devices are identical.
Nowadays this is not required because the system takes this data from the device descriptor - passed by the USB from the device through the mini-driver to the system.
A list of valid values is listed in file mmddk.h.