Handbook of Liturgical Practice of the Russian Church Abroad
For those appointed to wear it (which includes all Deacons and Priests), the ryassa — the wide-sleeved outer garment worn over the cassock — is always worn in the church unless it is superceded by the Priest or Deacon donning full liturgical vestments. Except in cases of exceptional necessity, a cleric never enters the temple at all during any liturgical or pastoral context without wearing his ryassa (i.e. wearing only his cassock); and most certainly, he never dons ‘lesser vestments’ (i.e. stole, cuffs and phelon for lesser services) merely over a cassock, under any circumstances, but always over a ryassa.
The exception is when a Priest or a Deacon enters the temple outside of a liturgical or pastoral context, in order, for example, to clean the Altar or perform other such works for which wearing a ryassa would not be appropriate.
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, at formal meals in a Parish Hall or similar venue, when speaking at a public engagement or representing the Church in a cultural or social setting, etc.; and all clergy always wear the ryassa in the presence of a Bishop, unless he invites them to do otherwise.