Handbook of ROCOR Liturgical Practice:

Is the epitrahil (stole) worn under the mantia, or over it?

When a monastic priest is in lesser vestments, the epitrahil is always worn over the mantia, not under it. Only an Archimandrite has the right to wear his epitrahil under the clasp of his mantia, and even this is not universal within the ROCOR.

This practice is not universally maintained elsewhere, but is consistent in our ROCOR heritage.