The rankings take into account a fighter’s wins/losses, quality of competition, finishing rate/dominance and frequency of fights.
Fighters are no longer eligible to be ranked after they’ve been inactive for 24 months, either due to injuries, drug/conduct suspensions, contract disputes or self-imposed hiatuses.
Fighters serving drug/conduct suspensions are eligible to be ranked, so long as they’re not inactive for more than 24 months.
To the best of our ability, fighters will be ranked in their primary weight class. Catchweight fights and bouts outside the fighter’s primary weight class can have a positive or negative impact on the ranking. However, non-titleholders can be ranked in only one weight class at a given time, and in most cases, they won’t be ranked in a new weight class until they’ve had their first fight at that weight.
Source : MMAJunkie