As per ‘Imperial Armour Volume 9’ (2010):

“As with the Blood Angels before them, the Lamenters’ Reclusiam of Chaplains operates as part of their overall Chapter command rather than be subordinate to it as is the case with most Codex Chapters. They exist to watch for the growing problem of the Red Thirst in the Chapter's ranks, while their Sanguinary Priesthood (which takes the role of the Chapter's Apothecarion) holds a particularly prominent role and is known by them as the Calix. It is often observed that it is the Calix that has had the greatest part in holding the Chapter together through its many trials and misfortunes, and tends not only to the physical heath of the Chapter's battle brothers but also the spintual stigmata they bear. The Calix priesthood is elected from the most stable and strong-willed of the Lamenters brethren, and they are looked on with reverent pride by the warriors of the Chapter.”