Prior to Vatican II there was no concelebration of the Mass by several priests. Therefore only one priest would be “on the altar.” But there are times (e.g. funerals) when several priests would want to be present but priests have a different role in the church than lay people so it would be inappropriate for them to simply be seated amongst the laity in the congregation. Such priests would “sit in choir” meaning they would be in a designated area and dressed according to their vocation and the nature of the celebration.
To be in “choir dress” means priests are dressed for public prayer (e.g. cassock ad surplice) but not for Mass (e.g. chasible). The particular form of the choir dress would depend any office(s) the priest might hold as well as whether he is a member of a religious community.
