Register and Device in ISU

Experts,

I see below info from F1 help about registers,

Device that measures consumption, energy, etc.

This can refer to an actual register or a display in an electronic device.

If register is device that measures energy that why do we have a separate concept called device and device category in ISU. How are registers different from a device in ISU. I am learning the concepts of ISU being a fresher. Requesting you all to kindly guide me.

BR,

Aspire