The PLX format is provided for footprint creation software to provide outputs for the import into Pulsonix footprint libraries and as such is not published. The footprint creation software normally defines how to create a footprint and export it into Pulsonix.
That exactly why I ask for it: I use PCB Libraries Library Expert Pro with Pulsonix export option (v2016.06) and it seems that PLX file they generate have some useless/confusing strings. It would be great to have PLX format description available so I could provide feedback to PCB Libraries on Pulsonix export.
The output .PLX file should just read into Pulsonix with the default set of Layer numbers provided by PCBLibraries and using our example technologies. Once within Pulsonix, layer data can of course be moved/changed. The receiving layers can be changed if wished to match your technology within their software as their Help instructs. If this is not working then we would like to know about this at the Support address already stated.