Tagging with projects would be great too, but we were really thinking of arbitrary tags not limited to the set of projects. (One example might be an "onboarding" tag; it's a word that's used in a few different contexts, so it would be pretty valuable to easily narrow in on just a subset -- e.g. pages intending to help new engineers onboard onto the team, which doesn't have an associated project.)

In T9622#141931, @rhkeeler wrote:

Tagging with projects would be great too, but we were really thinking of arbitrary tags not limited to the set of projects. (One example might be an "onboarding" tag; it's a word that's used in a few different contexts, so it would be pretty valuable to easily narrow in on just a subset -- e.g. pages intending to help new engineers onboard onto the team, which doesn't have an associated project.)

Full agree here with @rhkeeler. We're using this approach in other issue tracker/ wiki combos, too. Highly useful to semantically connect internal knowledge better.

Projects are "tags" in Phabricator, they are general purpose containers for describing groups of things including Projects, Tags, Teams, Buckets, Whatever.