Embed Interop Types

Added in .NET 4.0
This option was added to References > Properties
Instead of including these files as part of your solution they should be embedded into your assembly.
The PIAs are embedded into your project by default.
There is an "Embed Interop Type" property associated with each interop reference assembly.
If the "Embed Interop Type" property is set to False, you must include the PIA for each version of Office that the application will run with.

These can be embedded into your assembly so that you don't have to deploy them alongside your application
This means that my application is now independent of the version of excel that is installed on a machine.
This automatically includes all the PIA type references used in the project and therefore removes the need to install the PIA prior to deployment Include a screen shot of this for the Excel PIA
There is an "Embed Interop Type" property associated with each interop reference assembly

Microsoft doesn't publish the PIAs because the interop types should be embedded into the solution.
link - stackoverflow.com/questions/21013912/can-i-still-use-microsoft-office-interop-assemblies-with-office-2013