CV boot is a rubber barrier, i think BMW calls them bellows, that keeps all of the grease in the constant velocity joint while keeping all of the dirt out.
(CV ---> Constant Velocity) on the axle.

There is no need to touch the CV boot when changing control arms. You take the axle out from the hub if you need to. Think about getting the axle rebuilt depending on how long the boot has been torn. It is cheaper to replace the CV boot, but your mechanic might take the axle out anyways... Just because you replace the boot and clean it doesn't mean that the axle won't wear out in 3 months. Think about it.