Tiers are like, let's say, you have a mage character, then the tiers would be:

Mage 1 -- Mage 2 -- Mage 3 -- Mage 4 -- Mage 5 -- Mage 6

These tiers have general bonuses like 'Cooldown time down by .6 seconds' or 'Increase of 5 in Willpower' or things like that. Then, sub-tiers are 

Mage 2 has the Natural Magic and Necromancy sub-tiers, you select one after reaching said level and you'll be given a new ability, then, as you train said class enough, you can unlock more abilities from that same tier, and so on. 

Of course, this is an oversimplified example of what I had in mind, but if you want to add something new to it, then you are free to do so. Always open for suggestions! 

AND, @Thundercat, don't worry, don't worry, this ain't got nothing to do with either and those are just used as a referance so you'll be okay. :D