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  • Derrick Marshall thinks this is interesting:

Ability to share externally with or without authentication required. In SharePoint Online, you have the option of sharing content with external users. When you share a document with external users, you can require that they authenticate with a free Microsoft account or without any authentication. (The ability to do this is controlled globally and at the site collection level by the administrator.) When you share an individual document, you can also specify whether access is read-only or whether the external user can edit the document. If you do not require authentication and you allow a guest user to edit a document, the Modified By user shows up as Guest Contributor. If you require authentication, which means your...

From

Cover of Essential SharePoint 2013, Second Edition

Note

External user access