Handbook of Liturgical Practice of the Russian Church Abroad
When a monastic priest of the rank of either Hieromonk or Abbot (Igumen) 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.
Icon demonstrating the wearing of the epitrahil over the mantia, as is done by all Hieromonks and Abbots.
Icon demonstrating the wearing of the epitrahil under the mantia, as is done by exclusively by Archimandrites (and even this, not universally).
This practice is not universally maintained elsewhere, but is consistent in our ROCOR heritage and is to be maintained in all our monasteries and temples.
A contemporary photograph, in which can be seen an Hieromonk with the epitrahil properly worn over the top of his mantia.