How to explain path under /sys/bus/hid/devices

I am studying about HID driver, and there is one thing I cannot find out any document about it.Hope someone can help me.

When a HID device mount, I can found it on /sys/bus/hid/devices, it may looks like

0003:045E:0773.0002

Now, I known its format should be BUS:VID:PID.N.My question is, what is the N (in above example, 0002)? A HID mounting order? Or a index?And where can I find the document about this?Thanks.