We have been using Sharepoint Online External Sharing for some while now and have external (guest) users in our AAD. We have not used Azure B2B yet.
In our directory some external users have "Guest Migrated By Sharepoint" in their Department property. What does that mean?
Also some external users have "Microsoft Account" in Source property even when I am pretty sure they are using their Office 365 work account to login to SPO. Can this be? Some external users have "External Azure Active Directory as Source property.