You cannot debug or run this project,
because the required version of the
Microsft Office application is not
A shared Add-in is made from a Visual Studio project creating a Com Add-In implementing the IDTExtensibility2 interface. This grants you access to the Excel model (workbooks, worksheets, Range, …).
This kind of project is still today what looks the most like a C# XLA. They are often used for formal commercial development deployed to external clients. While they are more difficult to make and maintain, they do give you deeper control over the Add-in and can be made somewhat version-agnostic (that is, you can use your own IA instead of a specific Office PIA library, and/or can do more in late binding to handle multiple Office versions with lower risks of compatibility problems).