What is the <project name>.sqlproj.user file, and how is it created?

I want to move a .sln file to a directory above where it is currently located (because right now it's in a project directory, which isn't semantically correct). The project is called 'pName', and within the project there is a file pName.sqlproj.user. When i move the .sln file to the parent directory that same .user file is created. Why? is it due to which project is set as a startup?