The robes of the Dark Jedi Brotherhood are used as a means of visually identifying rank within the Brotherhood.
DJB Facts
Fiction
For many years, one could discern a member of the Brotherhood's precise rank by the raiment that they wore. This uniform of the Dark Side was found to be too stifling by the leaders of the brotherhood, and the decision was made to make more garments available to their Dark Jedi.
After many moons of extended clandestine negotiations with merchants and tailors across the galaxy, the Herald has acquired a great number of fine robes and clothing for the members of the Brotherhood.
Creation
Anyone in the DJB can use the Robe Construction Tool to create a set of robes for themselves based upon his/her rank.
A gallery of the available robes can be seen here. In the construction tool, one has the option to alter trim color, style and determine whether ot not they want an outer robe or cloak as well.
Upon reaching the Elder ranks one has considerable more options than the Journeyman ranks, and can even request a custom robe from the Herald.
As with Lightsabers the Herald has finally authority as to who may possess what robes.
Final Note
Members of the Dark Council and their staff may at times be granted custom robes regardless of their rank in the overall Brotherhood..