Every other office in this atlas serves a community that shares its faith. The chaplain does not, and the whole shape of the role follows from that.
A chaplain is placed in an institution, a regiment, a hospital, a prison, a ship, a university, among people who are there for reasons unconnected to religion, most of whom do not share the chaplain's own. The working expectation is that the chaplain serves all of them. In practice this means finding a priest for a dying Catholic when the chaplain is a Baptist, arranging halal provision, knowing which rites must be performed within hours of death and by whom, and sitting with people who want none of it.
The structural oddity is doubled authority. A chaplain is endorsed by a religious body and appointed by a secular institution, and the two can pull in different directions, over what may be said, over confidentiality, over whether the chaplain is present as a minister or as an officer. Military chaplains are commissioned in most armed forces and are non-combatants under the Geneva Conventions, which is a specific legal status and not a courtesy.
The office is no longer Christian by definition. Jewish, Muslim, Buddhist, Hindu, Sikh, pagan and humanist chaplains are endorsed and appointed in armed forces and health services in a number of countries. The resulting arguments, which bodies may endorse, whether a humanist can be a chaplain at all, whether public money should fund any of it, are among the more practical church-and-state disputes now running, and they are being decided institution by institution rather than by any general principle.