@Luis Bernardo Thanks for posting in our Q&A.
This setting is used to allow users to register their devices with Azure AD. Enrollment with Microsoft Intune or Mobile Device Management for Office 365 requires Device Registration. If you have configured either of these services, “ALL” will be selected, and the button will be disabled.
Currently, for tenants using the 1911 service release and later, the MDM authority is automatically set to Intune. So, this setting "Users may register their devices with Azure AD" is automatically set to "ALL".
Hope it will clarify something.
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