The Base ROM project is a special project type that contains the ROM to be modified. It is automatically created with the solution, and all mods projects in the solution automatically reference it. There is some internal support for having multiple Base ROM projects, but there is no way to accomplish this with the UI. It may be possible through manual project file editing, but even I, the author of Sky Editor, wouldn't want to attempt that. If you're interested in this feature after reading the rest of this tutorial, please leave feedback in this GitHub issue, and I'll prioritize it accordingly.
The reason one may want multiple Base ROM projects is for ROMs that are very similar, such as Pokémon Sun and Pokémon Moon. They're similar enough that some mod projects may be able to properly build against both. Such a setup would look like this:
- Sun BaseRom
- Moon BaseRom
- Generic Mod
- Sun Modpack
- Moon Modpack
When this build is complete, there would be two modpacks, one for each game.