I believe Wicked has something called an “onstage swing,” They’re job is just for immediate coverage for when an injury happens. This way the offstage swings can get time to get dressed and ready to replace whoever is out.
Other than that, that seems to be the accurate definition of a swing.
I’m a bit more curious about the difference between understudies/standbys when it comes to a show like Dear Evan Hansen. For example, the Larry Murphy cover is noted as a standby. While everyone else who covers a role are understudies. At first I thought it was due to the amount of roles they covered. The Larry Murphy cover just covers the one, while the others cover multiple roles. I believe it has more to do with contracts, though. If I’m not mistaken standbys are on principle contracts, understudies are on chorus contracts.
My father (AIDS)
My sister (AIDS)
My uncle and my cousin and her best friend (AIDS, AIDS, AIDS)
The gays and the straights
And the white and the spades