Sword Brethren are the Black Templars equivalent to the First Company of other Chapters. Black Templar veterans are elevated to this rank and placed in separate squads where they will fight as elites, train to use Terminator Armour (they are the only Black Templars allowed to use the armour) or form a Marshal's bodyguard. Because of this they are exempt from training Neophytes and unlike squads with Initiates, Neophytes will not be found in a Sword Brethren squad.
During Crusades, a Marshal can also appoint a Castellan to lead one of the Fighting Companies. This Castellan is picked from the Sword Brethren as is the case when a Castellan or Marshal is killed, the new one will be picked from the Sword Brethren.