Tiers were added to the CharGen system as a way of portraying the great fighters, the great masterminds and some of the rare and unusual concepts of the series. If we did not have Tiers, everyone would be setup equivalent to a Tier IV, which is designed to be above average and that is what we feel PCs should be. But we wanted to leave room for the Jaime Lannisters and Brienne of Tarths and we felt that it was easiest to manage by giving access to different things for each Tier rather than randomly adding more points to the setup of such a character.
Tiers were then tied to Type so that one component that decides the Type of a character is its CharGen setup, where Tier IIs and IIIs are given more points to set themselves up with as well as access to some attributes that IVs don’t have access to. A Tier II character is always at least Limited and a Tier III character is always at least Restricted. However, it is also possible for a Tier IV to be Limited; for example, the lord of a great house is always Limited, but that does not automatically qualify him for a higher Tier.
With this connection to Type, Tiers are essentially one component of what other games would consider “Feature” positions, though we use that exclusively for the few canon characters that show up in our family trees. Overall, Type and Tier simply means that it is possible for an experienced player to start out with the sort of role that otherwise might only be available through in-game promotions if they are willing to make the extra effort of submitting an application. If they prefer not to, Open characters are readily and easily available and they are not in any way second-class characters suitable just for newbies. Some of the key movers and shakers on the game are Open characters.