Federated authentication vs ABM for Apple Automated Device Enrollment

Daniel_K 

Federated authentication isn't strictly required if using a personal device but is typically used for accessing Microsoft 365 cloud services. With ABM you can also have "Managed Apple IDs" instead, but ABM isn't strictly required either if you want to just use Intune for BYOD.

The devices don't need a "plan" per-se, because they could just use Wi-Fi to enroll rather than 5G for  connectivity.

This gentleman has some good articles on ABM and Intune integration, which you may want to review:

How to enroll an Apple device with iOS 13 “User Enrollment” mode in Microsoft Intune 

How to start with Shared iPads for Business with Microsoft Endpoint Manager (Intune) and Apple Business Manager.

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