So what is a Revenue registration identity number?

In relation to commercial vehicle road tax (and associated thread), the declaration that will need to be signed requires that the signatory has a Revenue registration identity number?

What is a Revenue registration identity number?

Is it your PPS number?

If a person has a commercial vehicle and wants to be able to sign the declaration they'll need a Revenue registration identity number.

What is involved and how does it work?

It would be an off label use of the PPS number.