A Crossover person is someone whom you nominate to take over your duties on a temporary basis while you are unable to do so. For whatever reason: sick, on holiday, too much on your plate and you need help.
If you will be absent, you need to designate a Crossover person who can handle your duties in your absence. You cannot help being sick, but it is still your duty to ping someone and ask them to take over in your absence so that we don't drop the ball.
The Crossover person can make decisions on your behalf while you are gone. That's the default mode. If you do not want this, then make it clear.
The Crossover person must bring the original person back up to speed when they return. Inform them of all decisions made as well as all developments, so that continuity can be maintained. If you're the Crossover person and you do the work, but don't tell the original person, that's not very helpful. This means that you should be extra diligent with notetaking when you are acting as a crossover person.
You can nominate multiple crossover people. If you do this, be super clear in how you expect your duties to be split between the different crossover people.
Set an autoresponder for your email, and specify your crossover people.