Experience 2 (Source)

You cannot get mission higher then whatthey calculate your level ability to be - This can be seen with NPCs saying they have a mission and then after accepting saying they dont have anything for you. btw it is NOT tied to combat tiers ( ranged weapons) as a melee does not use that tier, Only ranged does that. Level increase based on your tier level in that weapon , Period. Beginning level is 5, Get Noveice brawler or marksman it goes to 6, Go to level 1 pistol or unarmed ( or any of the other tiers) it goes to 7, etc, It does not change if I also increase my ranged weapon support skill it stays at 7 until I move up another tree.