I'm sure that there used to be mention of this file in earlier versions of the SDK, but for the past few releases at least, there's been zero reference to this file.
My interest is primarily in the formatting and display of EIS panels. While one can of course work with the existing examples in the default aircraft (and look to the WT NXi aircraft addon configs for more), it would be extremely helpful for documentation to exist to outline the various possibilities open to us.
In addition, various avionics units have entries in the panel.xml, and those aren't documented anywhere that I can find. This again forces us to turn to implementations in existing aircraft for examples, instead of explaining the need and utility of these entries.