Hi @Marius - Roma ,
officially from Microsoft: https://learn.microsoft.com/en-us/microsoft-365/enterprise/subscriptions-licenses-accounts-and-tenants-for-microsoft-cloud-offerings?view=o365-worldwide#tenants
Also helpful: https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/active-directory-whatis
And a third one: https://blog.siliconvalve.com/2018/08/27/understanding-tenants-subscriptions-regions-and-geographies-in-azure/
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Regards
Andreas Baumgarten