So these were the initial semantics:
0 means "show to everybody"
5 means "show to other registered users"
9 means "show to nobody except maintainers"
Lars: yeah I don't say we have to copy them, I was just looking for inspiration, assuming that there might have gone some thought into it by aD already.
I think 'never show' and 'show to admins' is redundant, at least if you mean site-wide admins, since those could always look into the db directly anyway. 'Show to other members of subsite' makes a lot of sense though - I wonder if 'show to admins of subsite' would be needed too.
I think assigning it integers with some gaps in it for future or site-specific extensions would be good, as well as making it scalar (is that the correct word?) so that higher values always mean more restricted, with 0 as lowest value 'show to all', so that comparisons like $priv_email>$privacy_threshold will always work regardless of later extensions.