The Site option available in OpManager DE makes you to group all the Probes that are installed in a single location. Also with Site, it becomes easier to filter the devices when applying any configurations. First create a site and then create a probe and add to the site.
Steps to add a new Site:
From the Central web client, go to Admin-> Sites.
Click on Add Site.
Enter the Site Name.
Enter the name and the mail address of the contact person.
