On what constitutes/describes choir dress in general, my first thought is to look at GIRM 310 and 114 together. (I am looking only at the American GIRM, and I have not consulted the Latin.)
GIRM 114 foresees priests who are not the principal celebrant concelebrating, preferably, but if not, “they wear their proper choir dress or a surplice over a cassock.” On its own, this line can very naturally be read to mean that some priests (e.g., secular canons) have something called “choir dress” and that other priests have nothing called choir dress, and thus wear cassock and surplice instead of wearing some form of choir dress. CB 81 allows for the same kind of reading.
(This reading’s result seems odd to me, because cassock and surplice/cotta was the ‘generic’ choir dress for centuries before the Missal of St. Paul VI, but… things change.)
In light of GIRM 310, I think that the quoted line must be referring to “proper” choir dress vs. ‘generic’ choir dress. That paragraph addresses seating for exactly three kinds of priests: the priest celebrant, concelebrants, and “Priests who are present at the celebration in choir dress but without concelebrating”. There is no mention of a fourth kind of priests, wearing cassock and surplice, who are not in choir dress.
Consequently, I conclude that a priest’s choir dress is cassock and surplice, unless he has a different form of choir dress proper to him for some reason.
As such, CB 81’s reference to deacons in cassock and surplice seems to indicate that cassock and surplice should also be considered the ‘generic’ choir dress of deacons.
Looking further in CB, I see that CB 1209 gives chaplains of His Holiness the choir dress of the very same purple-ornamented formal cassock worn “on solemn but nonliturgical occasions” with only one additional item: a surplice. I think I have correctly discerned a certain step-down logic in CB 1206-1209, with the next step being, for ‘ordinary’ priests (who are not mentioned), an ‘ordinary’ cassock under that surplice.
CB 1199-1210 seemingly manages to specify the choir dress of every variety of bishop and priest other than the ordinary kind of priest… and the pope himself, so I guess we are in good company!
Perhaps CB 1199-1210 is now out of date in some way(s), but….
==
A: Good research! I’ll share these results on the blog for those who may find them helpful.