I was wondering if a creature who is immune to a condition, such as fatigue for a Horizon Walker with Terrain Dominance Desert, actually gains the fatigued condition, but does not take the listed penalties for it.
This seems to be a very important distinction for a Barbarian, in the case of fatigue and rage. Also for shaken or frightened, as their effects can stack to Panicked if they are not immune to that also.
This answer seems to say that you do gain the condition. However, this answer suggests that immunity to fatigue lets a barbarian enter and leave rage without worrying about that. The second answer is to a 3.5 question, so it could be something that changed between the two.