Hi,
If you already have licensed Exchange 2016 installed in the environment, and for CU23 it is not required or for any patches or updates.
Read the Pre-requisites for CU23 on Exchange Env - https://learn.microsoft.com/en-gb/Exchange/plan-and-deploy/prerequisites?view=exchserver-2016
If you want clarification on licenses check this - https://www.microsoft.com/en-gb/microsoft-365/exchange/microsoft-exchange-server-licensing-licensing-overview?rtc=1
Hope this helps.
JS
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