Hi, @Sebastien Doisy
Thank you for posting in Microsoft Q&A forum.
A service window is for a site server. A maintenance window is for a client.
So the client upgrade has nothing to do with service window, it is related to business hours and maintenance window, if you do not configure any maintenance window, the client upgrade will start at the end of business hours.
Client upgrades honor any Configuration Manager maintenance windows you've configured:
The more details about Business Hours and Maintenance Windows for your reference:
https://techcommunity.microsoft.com/t5/configuration-manager-archive/business-hours-vs-maintenance-windows-with-system-center-2012/ba-p/273032
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