User Layers/UPL disk space reclamation

If you have enabled App Layering User Layers or UPL in your non-persistent workspace environments, you are redirecting all of the user data over the network into remotely stored VHD files. Over time, as users write data into their image, the files will grow in size, consuming space on your back-end storage, which increases your total utilization and cost.

Now available in App Layering 2403 is an enhancement to enable user layer storage using the more modern VHDx container format. Moving to VHDx enables the ability to reclaim unused space within the user layer disk, returning it to the pool of available storage. User Layer space reclamation helps to decrease the footprint of enabling this powerful feature, reducing IT costs.

https://docs.citrix.com/en-us/citrix-app-layering/4/layer/enable-user-layers#user-layerupl-space-reclamation