This matrix was put together by my fellow PFE colleagues Kevin Kirkpatrick. You can view his blog here.
This matrix provides the 4 different external sharing options as well as the PowerShell equivalent to set them.
|Select this option:||If you want to:||Sharing Capability PowerShell Equivalent|
|Don’t allow sharing outside your organization||Prevent all users on all sites from sharing sites or sharing content on sites with external users. Users will not be able to share sites or content with external users, even if those users are already in your directory.||Disabled|
|Allow sharing only with the external users that already exist in your organization’s directory||Allow sharing only for external users who are already in your directory. These users may exist in your directory because they previously accepted sharing invitations or because they were manually imported. (You can tell an external user because they have #EXT# in their user name.)||ExistingExternalUserSharingOnly|
|Allow external users who accept sharing invitations and sign in as authenticated users||Require external users who have received invitations to view sites or content to sign-in with a Microsoft account before they can access the content.
In addition this Sharing Setting will allow for "New Secure Sharing* experience, secure links.
|Allow both external users who accept sharing invitations and guest links||Allow site users to share sites with people who sign in as authenticated users, but you also want to allow site users to share documents through the use of anonymous guest links, which do not require invited recipients to sign in.