If the sub deacon is ordained and is on track to the holy priesthood he may act as an assistant to the priest Celebrant at solemn Vespers and wears a cope as that is what the two assistants to the celebrant known as pluvialae wear during the celebration of solemn vespers. Other than that I wouldn’t know when a sub deacon gets to wear a cope.
Although in some places during traditional Pontifical solemn Masses the assistants (carrying miter, crosier, book and bugia) to the bishop and/or the faldstool wear a simple cope and these servers don’t have to be ordained men.
In the context of Holy Mass (Extraordinary Form) the subdeacon wears in addition to the cassock and fascia the amice, alb, cincture, maniple (if ordained) and tunicle (sacred vestment that is nearly identical to the deacon’s dalmatic but is usually shorter and has one less horizontal bar. The maniple is worn only during Mass and the biretta only in procession and when seated.
In summary a Sub deacon may wear a cope if he is an assistant to the priest celebrant at solemn Vespers and if he is acting as an assistant to the bishop at a Pontifical High Mass.
