In a .csproj file, what is <None Include="..."> for?

The MSDN article on the build action property explains the differences.

None - The file is not included in the project output group and is not compiled in the build process. An example is a text file that contains documentation, such as a Readme file.

Content - The file is not compiled, but is included in the Content output group. For example, this setting is the default value for an .htm or other kind of Web file.

One difference is how they get published; "None" items don't get included in a publish, "Content" items do; for example, on the "Application Files" dialog on the Publish tab.

Content files are not included in a build, but are included in a publish.

None files are not included in a build or publish, unless they are configured that way by you. For instance, a "Copy to Output Directory" setting of "Always" or "Newer", will cause them to be included in both a build and publish.

In my case .Pubxml is one of those files among None list. It's not meant for solution building or as a static file for web project. But to publish the site to Azure, the configurations are present in this.

As per Microsoft article these are the major types we see among .csproj file tags:

None - The file is not included in the project output group and is not compiled in the build process. An example is a text file that contains documentation, such as a Readme file.

Compile - The file is compiled into the build output. This setting is used for code files.

Content - The file is not compiled, but is included in the Content output group. For example, this setting is the default value for an .htm or other kind of Web file.

Embedded Resource - This file is embedded in the main project build output as a DLL or executable. It is typically used for resource files.