Earlier quoted context omitted.
Had a briefing with our company lawyer a while back and any information can be considered PII when paired with other information. Eg that you bought 7 foo’s is not PII, but that you bought 7 foo’s on Tuesday might be if that can then be looked up in the purchase history and you were the only one who bought 7 on Tuesday.
Does it have to be uniquely identifying to be PII? Or is there some minimum threshold for k-anonymity?
It just has to be correlatable if I understood it correctly, but I don't know if unique or not. To me it sounded like if there's only a small number of possible people it could identify (say 4) then its potentially PII, however I have no idea where the line is drawn. Clearly if k is 1, its PII. If k is 2, it probably is too. If k is 1000, its probably not. But at what point does it stop being PII? I have no idea!
The legal person basically said "its complicated, anything can become PII when combined with something else, even if neither on their own are PII". The bottom line is does some combination of information identify a person, then its PII (its in the name really!), but unfortunately that means there is no clear simple list of things that are or aren't PII, it really depends on each individual case.
Her advice was to think carefully about any data stored about or for users and to avoid storing it if possible, and if not possible, think carefully about whether or not it could identify a user in some way. Its not a very satisfying answer, I know. It also doesn't answer your question :(