Any type or number of devices can be tagged or added to a group. A group is a selection of devices that share characteristics, location, end-user types or pretty much anything you can think of. 

For example, you would group all devices in the same building into one group so they can be easily identified. You could also group devices by end-user type, for example, one year group may use a specific app like reading eggs that will need to be on every device. 

Therefore you would group all of these year group devices together so you can deploy apps, settings and other repositories with a click of a button.