Handbook of ROCOR Liturgical Practice:

Does a Hieromonk wear his mantia instead of a phelonion, or together with it?

In terms of ‘lesser vestments’, the ROCOR maintains the old Imperial practice of both the mantia and phelon being worn together on most occasions (during, for example, a Vigil service). The Hieromonk’s general attire for ‘lesser vestments’ is his ryassa covered by his mantia, over which he wears the epitrahil (stole) and cuffs. When the service calls for the phelon to be added, this is placed over the top of these other garments.

In present-day practice in other Russian Orthodox traditions, the mantia is removed whenever the phelon is added; but the ROCOR maintains this older practice of wearing both.