Handbook of ROCOR Liturgical Practice:

When is the ryassa to be worn?


For those appointed to wear it (which includes all Deacons and Priests), the ryassa (the wide-sleeved outer garment that is worn over the cassock) is alwaysworn in the church unless it is superceded by the Priest or Deacon donning full liturgical vestments. Except in cases of exceptional need, a cleric never enters the temple at all without wearing a ryassa, for any reason; and most certainly, he never dons ‘lesser vestments’ (i.e. stole, cuffs and phelon for lesser services) merely over a cassock, but always over a ryassa.

The ryassa is also worn by the Priest and Deacon in all formal circumstances of priestly life. It should be worn, for example, when teaching a parish class, when chairing a parish council meeting, etc.; and all clergy always wear the ryassa in the presence of a Bishop, unless he invites them to do otherwise.