Rule Engine helps you automate repetitive tasks whenever a new device is added to OpManager. It allows you to automatically add monitors, associate notification profiles, add devices to a business view, and more. A rule has criteria and a set of actions. Once the criteria is fixed, OpManager applies actions to the devices that obey the set of rules. You can find Discovery Rule Engine under Settings -> Discovery -> Discovery Rule Engine
You will be directed to the following page. From here you can choose to either rerun, add or edit rules.
If you select the "Add Rule" option, a new window will appear asking you to enter the name, description, criteria, and the action to be taken. Click here to know about configuring discovery rule engine in OpManager.