Vedic sacrifices like the Somayāga comprise elaborate rites needing 16 priests in all: the four principal priests, each having three assistants. Brāhmaṇā-cchaṁsin is an assistant of the fourth principal priest called Brahmā.
In the Agniṣṭoma sacrifice (which is a model sacrifice in the Somayāga group) special recitations called ‘ājya-śastra’ have to be chanted, during the first pressing of the soma juice. They are four in number. Brāhmaṇācchaṁsin chants the third recitation addressed to Indra.