Tag said: "It means the stage manager asked them to step away from the show that day so an understudy/swing can practice their track.
That doesn't really happen, as it costs money.
"
Actually, this is incorrect. On Broadway (and in any other union house), a swing is on a weekly salary, the same as other company members. The "swung out" company member receives their regular pay for watching the show and noting roles s/he covers; the swing receives their regular pay as well.
The only time that a swing's performance costs a company additional money is when the swing goes on in a leading role.