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For MSIX Apps, you don't need App Masking. If you want to remove application for an user, remove user from AVD app group so that app will be disappeared.
However, you need to create multiple app groups for multiple applications.
I checked with internal team on this, there is a feature request in backlog that would allow for assigning of different apps to different users in the same host pool for MSIX App Attach. There is no ETA on this yet.
In the meantime App Groups for your grouped apps is going to be the best method. It shouldn't involve a significant overhead if you use group membership to assign various apps, or groups of apps to their target users.
Hope this helps.
If you need further help on this, tag me in a comment.
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