I have been looking for a plug-in to replace Cross Reference Organizer for X-ref of other .skp files. I use Layers Panel and create a lot of layers, so big models can become unwieldly with all of the layers and information, plus it's much more convenient to work in sub-models. Being able to X-ref in another .skp file, which is held in an easily deleted container if required, plus having notification in the main model of updates in the sub-model is very similar to Cross Reference Organizer's notification of same. I have tested several working procedures and it all works great!