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A tab displayed in Salesforce that is an external web application with authentication
Mashups
It’s like something back from the late 00’s when Yahoo Pipes was a thing (yes Pipes Digital is a thing, and so is Huginn)
http://sfdc-know-how.blogspot.com/2014/03/mashup-integration-in-salesforce.html
Client Presentation - visual layer only
Client Service
External Service
Eg embedding an iFrame into Salesforce (syncronous)
Or linking to an external system with a deeplink (asynchronous)
Web Tab