Rogue Set
From Wiki
A set of 4 obtained by stealing and as reward packs as part of the Thieves Guild System. These drop in Schematic form, and at a very rare rate.
Levelable
These unique items, when worn as a set, will gain a certain amount of experience points when a thief task is completed. When a piece accumulates enough points, it will gain in a level, not to exceed 4. Each level will give a boost to the existing set bonuses that each has.