Can somebody please tell me what the following icon means in Visual Studio 2017?
This is a Xamarin cross-platform plugin project. This icon appears next to a class in the platform specific projects. When clicking on the filename, Visual Studio opens up the same named class in the cross-platform class library project in the same solution. So it appears that the platform specific projects all have a copy/link to the same class in the cross-platform project.
The specific example of this is located in the project located at https://github.com/jamesmontemagno/LaunchMapsPlugin. The file (CrossExternalMap) does not exist in the platform projects until the solution is loaded into Visual Studio.
It appears it is just a file link to an existing item by doing Add->Existing Item->Add as Link as shown below:
https://andrewlock.net/including-linked-files-from-outside-the-project-directory-in-asp-net-core/