I have reached out to internal AVD team to cross check on your query.
The RDS CAL clearly states within Microsoft Learn as needed for the operating system used by the end-users. Users should review if a server operating system is needed for this host pool or if they can use win10 multisession.
If you use a Server operating system as host pool in AVD you have to build a RDS license server with CAL.
If use use windows 10 or 7 it is included, so always try to prefer windows Client instead of windows server operating system.
https://azure.microsoft.com/en-us/pricing/details/virtual-desktop/
Hope this helps.
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