In the standard 3:16 rules the PC with the highest Kills levels up automatically and the rest rolls to see which other PC levels also. Only 2 PCs can level between Missions.
In AD316 Gregor introduces Tokens (Accomplishments) as an additional way to level. In the Development phase the PC with the highest Kills and the one with the most Tokens roll to see which one levels. After that all the other PCs that didn’t level roll to see which one also gains a level. There is still the limit of 2 PCs levelling between Missions.