A Boolean that you can use to control the output of debug messages for a particular debug zone.

You can set debugging zones in the registry on the development workstation.

The following example shows how to associate bit 0 of a mask bit with an INFO zone.

#define ZONE_INFO   DEBUGZONE(0)

In this example, ZONE_INFO could be used to as an argument to DEBUGMSG to provide a conditional debugging message.

Reference