Quick question - does anybody know whether or not new layers can be created yet? I can't even find where the existing ones are defined. Any clues are appreciated!
Most of time you can't just make up values for units - the game has fixed lists of them will typically ignore anything else. The best resources for discovering the options is probably multi-file search for the key name; there has been some documentation here and there, but it might be out of date - PTE just added an underwater layer for instance. I've been putting modding stuff on PA-Wiki if you want to try and improve the documentation a bit. http://pawiki.net/wiki/Units_(Modding)
I believe the existing layers are defined by the C++(?) code that's compiled to create the executables. In other words, they're unmodifiable by us.
@wondible @stuart98 Yeah it looks like this isn't really possible yet. I was hoping there was a simple way to define a new air layer and then assign an altitude value for it, like AirLayerLow:100, AirLayerMid:200, AirLayerHigh:300 ... or something like that. I was hoping to at least be able to back the orbital layer up off the planet a bit more, but I think that's on hold too for now.