According to NOAD, it means
system 3 Music a set of staves in a musical score joined by a brace.
Etymonline gives its history as
1610s, "the whole creation, the universe," from L.L. systema "an arrangement, system," from Gk. systema "organized whole, body," from syn- "together" + root of histanai "cause to stand" from PIE base *sta- "to stand" (see stet). Meaning "set of correlated principles, facts, ideas, etc." first recorded 1630s. Meaning "animal body as an organized whole, sum of the vital processes in an organism" is recorded from 1680s; hence figurative phrase to get (something) out of one's system (1900). Computer sense of "group of related programs" is recorded from 1963. All systems go (1962) is from U.S. space program.