Except:
PIN is an acronym for Personal Identification Number. The DVLA is a government agency. Their web site (https://www.direct.gov.uk/taxdisc) must be used by thousands of people every day.
Is it that the person who wrote the words in that box didn't understand how acronyms work, or that they did but they put it there for the many stupid people who must access their site daily who don't understand what a PIN is unless it has the word "number" afterwards?
I emailed them a complaint; I'm sure they have a suitable computer-generated reply they'll send me back.