MDM User scope is used to define which USERS will be able to enroll their devices into Intune as part of automatic enrollment (with MEM CM for instance) or using Azure AD join on W10.

Everythning is documented there : https://docs.microsoft.com/en-us/mem/intune/enrollment/windows-enroll

Additionnaly, watch this video which explains the differences between MDM and MAM service management URLs : https://youtu.be/ZwmilvZEIUw