The file contains a reference to the help file schema "http://schemas.planatechsolutions.com/xlp/601/help". This schema was installed as part of the XLL+ tool-kit; as a result syntax checking, intellisense and popup help are available in the Visual Studio XML editor.
Intellisense helps a lot with editing an XML file. All the legal options are listed in a dropdown as soon as you start typing, so you don't need to learn the file format.
Syntax checking is also useful, since it catches any syntax errors right when they occur.
Popup help for each element and attribute explains what is required, what is optional, and what is the purpose of the element.