[robertlagrant]

Fair point on PII.

But to the rest of your post: I don't think so. I think there an identification number is something like a government-issued ID number.

An online identifier would have to identify you (e.g. my Hacker News username is probably identifiable).

The way I think of it is if someone who isn't authorised to know who I am can look in a system at the number and then go off and correlate that info to find me without further reference to other data in said system.

A database ID doesn't count, because you'd then need to look up something actually identifiable in the system to figure out who I am; neither does an opaque tracking number.

My social security number is identifiable; my email address may be identifiable; if I gave birth in region X to octuplets, then that probably is too.