For formal occasions (for example, in the heading of a letter or when introducing a speaker), one would politely address or refer to a subdeacon as "Subdeacon [John Smith]." In informal settings, for example, in normal conversation, it is appropriate to simply refer to a subdeacon by his first name.
Sacramentally, all subdeacons are equal; if there are multiple subdeacons, they are ranked and serve by seniority according to the date of their ordination. Additionally,
subdeacons are not able to give blessings.