Hello,
Q1: When using a Per User CAL, the CAL is allocated when someone logs into the server. This means the CAL is assigned to the first user who logs in, until all CALs are allocated.
The Per User CAL information is stored in your domain, so a domain is necessary if you wish to use Per User CALs.
Q2: It is possible. Each server can specify a license server in its LOCAL POLICY. There's no need to configure them in GPO, as the local policy would be overridden by GPO.
You can find the setting in the documenthttps://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/rds-license-session-hosts
Q3: Correct, if you have more than 10 users, you should purchase additional License CALs.
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