In the Eastern Orthodox Church at least, Justinianus, subdeacons are in the order immediately below that of deacon. IOW, subdeacons are *not* deacons, but they function almost like acolytes. At Pontifical Liturgies, they are specifically charged with assisting the presiding hierarch in vesting in the middle of the church.
That being said, the diaconate does have a hierarchy of its own, so to speak.
First, there are the simple married deacons who wear a single diaconal orarion (diaconal stole). Then:
"Hierodeacons" are deacons who are also monks.
"Protodeacon" is an honorific title given to married deacons after a certain term of honorable service. In the Russian tradition at least, it comes with the privilege of wearing the double-orarion.
"Archdeacon" is the title given to the principal deacon of a monastery or one attached to a bishop. If the latter, he is usually celibate, but this is not an absolute requirement. In the Russian traditon, an archdeacon, in addition to the privilege of wearing a double-orarion, is also invested with a purple kamilavka (cylindrical head-covering) if attached to a bishop. A metropolitan's archdeacon wears a blue kamilavka, and the Patriarch of Moscow's archdeacon(s) wears a green kamilavka.
I believe that among the Greeks all deacons wear the double-orarion.