The priority of DCP channels is affected by the outer ring on the DCP panel track ball. Primary changes are always mixed together, there is no hard priority. Turning the outer ring down will mix the primary change into the background and thus behind another channel. Secondary changes are normally also transparent. By default there is no priority, so the changes made to two overlapping areas will combine together. This is the normal mode of operation and is ideal for ESR area isolations.
If the outer ring is turned down by a small amount then all
transparency will be removed, giving a rigid priority. This is useful
if a channel is to be used for colour isolation, rather than area
isolation. The colours isolated by higher priority channels, starting
from red, will not be affected by lower priority channels. For example,
this allows several objects to be isolated and then the rest of the
scene can be desaturated.
Obviously, all of this only applies where layers overlap.