Update device-update-groups.md

Author: vimeht

articles/iot-hub-device-update/device-update-groups.md

@@ -64,9 +64,17 @@ Below are the devices and the possible groups that can be created for them.
 | Device1 | Group1                        |
 | Device2 | Group1                        |
 | Device3 | Group2                        |
-| Device4 | DefaultGroup1-(deviceClassId) |
+| Device4 | DefaultGroup                  |
 
 
+## Subgroups
+
+All user-created groups as well as default groups are automatically sub-categorized into one or more subgroups to allow administrators to manage heterogeneous devices in an organized and efficient manner, by providing a way to perform updates on a set of devices that share compatibility properties. Each device class within a group maps to one subgroup. Devices are added to a subgroup based on their compatibility properties and the Device Update PnP model ID, which together comprise a device class. 
+
+A group can have one or more best available update, one for each subgroup it contains. 
+
+[Learn more](https://learn.microsoft.com/en-us/azure/iot-hub-device-update/device-update-configuration-file) about setting compatibility properties on the Device Update agent
+
 ## Next steps
 
 [Create a device group](./create-update-group.md)