The subdeacon's essential role is to assist the bishop during a hierarchical Divine Liturgy (a Divine Liturgy at which a bishop is present and presiding) by vesting him, holding his service book, carrying his staff, presenting him with the dikiri and trikiri, etc. There is a special service for the ordination of a subdeacon, although in contemporary practice an acolyte or a reader may receive the bishop's blessing to vest and act as a subdeacon, either for a particular occasion or permanently. The main reason for this practice lies in the fact that the canons (e.g. Apostolic canon 26 et al.) prohibit subdeacons to marry after their ordination (just like deacons and priests). This latter stipulation has sometimes led to the reservation of the formal ordination service to candidates for the priesthood, although this is not universal.
The subdeacon is vested in a sticharion with an orarion tied around his waist, up over his shoulders (forming a cross in back), and with the ends hanging down in front, tucked under the section around the waist. Like readers, subdeacons are permitted to wear a cassock, although many only do so when attending services.
When there is no bishop present, a subdeacon will take the role of acolyte, assisting the priest during religious services in the altar.