In a .csproj file, what is for?

Question

How is

different from

?<

Answer 1

I am not 100% sure (I read the MSDN description of Build Action property) but just copying that answer from MSDN to StackOverflow does not answer the question completely for me.

The difference of None and Content only has an effect on Web projects. For a command line project, WinForm project or UnitTest project (in my case) etc. None and Content have no different behavior.

MSDN: "project output group" or "Content output group" only terms used in a Web project, right?