Actually, it's a complex TLA because it contains within itself another TLA, namely USB. It is this shortening of the instantly recognisabe term USB to the solitary letter U that makes the use of UPD such a poor choice, often requiring clarification, as already witnessed here on several occasions.It is a simple TLA, needing no more nor less explanation than any other acronym
However, you can take a horse to water but you can't make it drink, so for the sake of forum harmony I will not longer attemp to dissuade BH from his insistence in using this term.
Nuff said?