[Richard]

The purpose of the switch is to add or subtract weight from the tag. The switch can be used to add weight to the tag and subtract weight from the drive when passing through an Ohio toll booth where the axles are automatically weighed. I don't know if it's still true, but we used to be over the Ohio limit for a single axle.
It can be used to remove weight from the tag when turning a sharp corner or to add weight to the drive for traction. I actually used this once when the coach spun out on a snowy uphill.

The tags and the drive air bags are definitely hooked together. However, the switch activates a couple of three way valves that allow you to isolate the tag bags from the drive bags.