- it has to be written for a cappella performance by a mixed soprano-alto-tenor-bass choir. It can't have a mandatory instrumental part (e.g. piano, organ, string quarter, etc.), but may have an optional instrumental part, e.g. it could be a Choir + Organ piece where the composer has explicitly stated (e.g. in the sheet) that the instrumental may be dropped for some reason (e.g. if the choir is large enough) - these are acceptable.
- it has to be an original Mozart work - KV catalogue index and all. Arrangements of Mozart works by other composers (e.g. a cappella arrangements of Requiem) don't count.
- it may require a solo performer.
Basically, imagine Mozart hearing Viadana's Exultate Justi, saying to himself: "I can do better than that!" and getting straight to work. I'm searching for the result of that!