Rookie class

Rookie (Sometimes referred to as middle) refers to the often third state of evolution Mon normally go through during their lifespans. Upon achieving this form, most Mon develop limbs, if they didn't have any before, and are more mobile than their other forms, getting access to stronger attacks compared to their smaller states. As a Rookie, Mon develop the base body of all it's digivolved states, except on abnormal states, though it is known that some Mon change radically their own appearance after evolving. Without taking in account abnormal evolutions, all higher states of a single Mon will share at least its condition as a virus, vaccine or data (or other attributes) with its Rookie form. This is considered as the "common" state a Mon can be in, and most Mon stay at this form. Due to this, there are more Rookie Mon than any other evolution state.

Partner Mon for Tamers often revert back to their Rookie state after digivolving successfully and, most normally, winning a battle. This isn't necessary, but most Tamers revert their Mon back to their child states for various reasons. (Depends on the duo) It is not necessary for a Mon to revert back, but most Mon do due to the stable memory space the Rookie state can constantly and accurately maintain.

Once a Child Mon reaches the Rookie level, it becomes difficult for them to revert to previous evolution levels again. In addition, as of the series, it has not been a case of Digimon reverting to their Child forms without having to turn them into a Eggs in order for it to hatch again, even though they seem to revert to their Child form for various reasons.