You use markers to associate your desktop assignments with specific image versions in an image stream.
The following figure depicts the initial state of an image management scenario.

In this scenario, several desktop pools/assignments use different versions within the Win10CorpKnow image stream. The administrator uses markers to associate specific image versions with specific pools/assignments. For example, the administrator has provisioned Pool 1 to a user group dedicated to user acceptance testing. By tagging both Pool 1 and image version 19 with the UAT marker, the administrator ensures that the desktops in Pool 1 are cloned from image version 19.
Now suppose that the administrator wants to provide the user acceptance testers with a modified desktop image. To accomplish this task, the administrator creates a new version 20 in the image catalog. Then they customize the underlying image on the source pod and publish the customized version 20 to all the other pods. Finally, they reassociate or move the UAT marker from version 19 to version 20, as shown in the following figure.

By reassociating or moving the UAT marker, the administrator redefines the image used for Pool 1.
- For Horizon pods, the marker reassociation triggers an automatic process in which all the desktops in Pool 1 are updated with image version 20.
- For pods in Microsoft Azure, the marker reassociation prompts Horizon Image Management Service to first validate whether Pool 1 is online and ready to receive an image update. If the validation is successful, Horizon Image Management Service proceeds to update all the desktops in Pool 1 with image version 20.