What are the current reliever fatigue rules?

Sorry if I missed this posted somewhere but can somebody point me to fatigue rules for relievers?

I had R5 Dixie Howell pitch 4.1 innings in game 1.
In game 2 my starter went the distance.
In game 3 Howell was the last reliever used in an extra inning game and came in fatigued at F4?

I thought extra rest only applied if they pitched above their endurance rating? Can someone explain?

Now it is a black box setting that takes into account how many pitches they make in a given appearance as well. High pitch counts sometimes require relievers to get 2 days rest to recover to F9 again.

Pretty sure it's first team to 20, but hey no super reliever.