QVS file which generally say Qlikview Settings file is an simple file people maintain to create the GLOBAL variables to store the locations of QVDs and any other files which are used in your application.
There are no hard rules for creating this files. mostly you can think of these as INCLUDE files (if you've used in any of your projects).
As mentioned already, QVS file extension is commonly used to indicate a text file with Qlik script content. Instead of repeating common content in many apps, you can centralize script to a reusable file.
See $(Must_Include=filename) as reference on including a script file inside a script.