Digimon level is start from 1 and up to level 99. It is devide to Rookie, Champion, Ultimate, Mega, and Burst Mode.
Basicly level 1-10 is Rookie, level 11-20 is Champion, level 21-40 is Ultimate, level 41-65 is Mega, and level 65(71 for partner digimon) is Burst Mode.
But some is digimon start from Champion Class at level 1. And there is some digimon that did not have Burst Mode now, maybe will be released someday. Look at this table :