The surface the link was on: 'help', 'notification', 'sync', 'drops', ...
Referrals only: the server that was joined.
Only a vote has one. A click is an unauthenticated redirect, so the server never learns who followed it - null here is the real answer, not a missing field.
Null when the id was not in any cluster's cache, which is not the same as having no user.
What happened: the button type for a click, 'forced'/'voluntary' for a vote, 'join' for a referral.