The editor used to edit a given file extension can be configured on this page. This makes it possible to configure the application to use a given editor for bespoke file extensions. i.e. say your project uses an xml file with the extension ".myProjectFile". It's possible to configure Liquid XML Studio to load this using the XML Editor.
Selecting a file extension and pressing the Editor button (or pressing the Add button) allows you to edit/create the file extension to Editor mapping.