A “server” is an altar boy (or a seminarian, or, sometimes, a member of a minor clerical order like a subdeacon) who assists the priest celebrating the Catholic Mass. In Hades, Bloom watches as "A server bearing a brass bucket" precedes the priest into the mortuary chapel, and then listens as the boy sings the Latin lines that the liturgy prescribes as responses to the priest's: "The server piped the answers in the treble." Stephen has performed this role in his youth, and in Telemachus, taunted by Mulligan's adoption of the role of a priestly celebrant, he feels himself back in that subordinate position.

JH 2011

Source: www.chnetwork.org.