The engineers at Stan's built off the success of the wildly popular Arch rim to bring us the Arch EX. It is lighter and stronger and offers the same rock solid performance and durability we have grown accustomed to with the Arch series rim. In general, the Arch EX is built for a variety of riding styles. It is light enough for XC riders and durable enough for all-mountain and trail riding.
You cannot discuss Stan's rims without talking about BST or Bead Socket Technology. With BST, the sidewall of the rim is shortened and the bottom of the bead rests directly on a wider platform. This means the bottom of the bead is supported by the base of the rim, not by only a UST channel carved into the sidewall of the rim like other manufacturers. It might sound like a small detail, but it makes a massive difference. By lowering the sidewall of the rim mechanical leverage the tire has on the rim is reduced. This will make the rim stiffer and last longer by eliminating flexing that can cause cracking. The BST design will allow you to run lower pressures and use 4mm more of the tire than ever before. This gives the tire larger air volume giving you better traction versus any other rim in the world. It can be used with or without tubes.
The other phenomenon that occurs when you lower the height of the sidewall is the reduction of weight. This explains why Stan's rims are much lighter than the competitions. It is one of the few documented cases of where lighter is also stronger.
Please note: Stan's BST rims are not compatible with WTB TCS and some UST specific 29" tires.