The "places for clergy" refers to a "choir" or in this context, simply a special seating section for other clergy who are watching a religious ritual. When it refers to bishops and priests At----The---Altar, it refers only to the one priest or bishop, or the several (2 or 3) that are actively involved in performing the ritual. The "choir" refers to the gallery of clergy who are not actively participating, but simply watching the ritual of the Mass being performed. The distinction here is rather technical. There might be, for example, 1, 2, or 3 Bishops or Priests actually performing the ritual to Consecrate the Bread and Wine. They alone would be AT---THE---ALTAR. The other members of clergy would be observing. Their participation would be to stand, sit, or kneel at certain times, and recite or sing at certain moments of the ritual. So, The---Places---Where were found in all cathedrals; but they are all distinct and different places. If this explanation is not adequate, let me know.

I think that 'where (bishops and priests could celebrate mass)' is just referring to the altar. Mass is celebrated by these people from the altar, not from the choir.