An instance property applies to a specific system server. When you create both a global value and an instance value for the same property, the instance value takes precedence.
You can configure the mxe.crontask.donotrun system property to be an instance-specific property by specifying a specific server, such as MXServer1. You specify a value that applies only to that server, such as the bulletin board cron task, BBCron. As a result, BBCron does not run on MXServer1. However, BBCron can run on another server instance, such as MXServer2.