Full servers: based on total population or current players logged in?
Solution 1:
Official source from 'CC Eva', a community coordinator:
when you see a server FULL means that the people who’ve chosen that server as theirs, or players who have moved to that particular server, have reached the cap, independently of where did they create their accounts.
This means that the number of accounts with the server as their home server has reached the capacity cap set for that server. Over time if people move away from the server it will become available again for new accounts to choose as their home server.