I can't believe I never asked this, but a friend asked me and I couldn't find something definitive. My search through the forums didn't give me an easy answer. (It might have been answered, but I couldn't find it.)
In the Powerhouse archetype, there's an immunity block for 44 points. I'm wondering about the Immunity Fatigue item. Immunity from fatigue effects is 5 points (under Immunity). I can account for 43 points in the stat powers:
Enhanced Stamina: 20 pts.
Impervious on Toughness: 12 pts.
Immunity (Cold Damage): 5 pts.
Immunity (Fire Damage): 5 pts.
Immunity (pressure): 1 pt.
Which means that Immunity (fatigue) is 1 pt (or the math is wrong, but you guys would have caught that).
So clearly it's meant to be Immunity (fatigue) 1 pt.
Here's what I think it means: the player is immune to the fatigue condition, but not the other fatigued effects. So the Powerhouse can (for example) use Extra Effort with impunity because it only causes Fatigue...but if the Extra Effort is used with something else that causes Fatigue, the conditions stack and the character is exhausted. Or if an Affliction causes an exhausted condition, well, that affects the character. (On the other hand, because you remove the worse fatigue effect first, the character recovers faster than others: only two conditions to remove.)
Been a while since I played, so I might have something else wrong in my explanation.
Does that sound right to you?