Tiers are based off of stats, DPS and utility
SS+ and above are allowed in the Tier list, nothing below said rank will be involved unless the Digimon is that significant.
D Tier - Digimon that are kind of terrible and has better option for less or same costs.
C Tier - Digimon that are decently viable for running early game dungeons.
B Tier - Digimon that are the best SS+ has to offer, could be used in most mid game content with the right setup.
A Tier - Digimon that are good enough to run most end game contents, but clearly has way better options. Each one of them suffers from some kind of weakness that's stopping them from being used as a staple.
S Tier - Digimon that are pretty much recommended to use for any end game content the game has to offer.
SS Tier - Digimon that are the best of what the game can offer for respective contents.
SSS Tier - Digimon that are the best and current top favored meta.
You might note that Ordinemon is not listed upon any of the Tier lists, well that's because there is more or less only one UN digi of any notice, which is her. She is in a class of sorts of her own being as versatile as she is being good vs everything and being SSS digi ranked. The other two being SS and SS+ Rank Apoc Psy and Arma Conflation, thus making a whole bracket for three digis one being in SSS and the other more or less being C or D isn't much worth.
Her power rivals that of the SS and SSS tier brackets here in this post currently.
For a list of all Digimon sorted by their in game rank click here and you can peak at any Digimon sorted by their ranking.
Tier List by Typing