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S4S lets you expose Salesforce Campaign Members as Sitecore users by setting up a separate domain for this provider. This functionality is provided for purposes such as reporting, emailing, etc and as such, these users are read-only and are not used for authentication purposes
Security Connector for Account Lookup
Adding this functionality allows your customers, represented in Salesforce as a custom object with a lookup relationship to a parent Account, to login to your Sitecore website using credentials stored in their custom Salesforce record.