I just answered a separate support request (posted after yours) about what the .skyl file is here:
Generally, we don't recommend trying to send these files by themselves for the purpose of sharing a Skyline document/project. Instead you should either use: 1) File > Share, 2) File > Upload to Panorama. Both will package everything necessary into a ZIP file. I also use File > Share to make a handy snapshot of the entire project at a certain state. The ZIP file contains everything, including spectral and iRT libraries and when you then open it with File > Open the files are extracted into a separate folder, which you can do any number of times, allowing you to start from the same snapshot over and over.
Hope this helps. Thanks for posting your question to the Skyline support board.