Silent Fury

The notation in brackets that is included as part of the Silent Sisterhood (X) special rule defines which Chamber Militant the model belongs to. This Chamber Militant may grant the model some additional rules or effects and influences how it may be selected when building an army that incorporates a Detachment with the Sisters of Silence Faction. 
When selecting a Detachment using the Sisters of Silence Army List, the number of units of Chambers Militant other than the Chamber of Vigilance is limited. For each HQ choice with a given Chamber Militant, the Detachment may include up to three other choices of the same Chamber Militant (except for units with the Chamber of Vigilance variant of the Silent Sisterhood (X) special rule, of which an army may include any number, limited only by available Force Organisation slots). Note that Dedicated Transports and units selected using the Retinue special rule do not count against these limits regardless of their variant of the Silent Sisterhood (X) special rule.
For example, a Sisters of Silence Detachment includes three HQ choices – two Knights Centura (Chamber of Oblivion) and a Silent Judge (Chamber of Judgement). This army may include any number of units from the Chamber of Vigilance, up to six units from the Chamber of Oblivion, and up to three units from the Chamber of Judgement.