Switch among multiple manuscripts in same Scrivener project file?

Another approach is to make use of the default subdocument section type feature, which can be applied to groups, and impacts everything below that point in a cascading fashion. In other words you can have a main group in the Draft folder for the novella, and set it’s default Section Type to “Chapter” for example. Now everything you put into that group will be a chapter by default. If you want more structure, you can set sub-groups to have their own defaults. So in short you can have an area of the Draft that completely follows its own rules, ignoring the Project Settings.

To play with this idea, right-click on the group that should be different, and from the Section Types submenu in the contextual menu you will see the Default Subdocument Type section that I’m referring to.

Overall I would say Scrivener has a lot of support for multiple works in one project, and it doesn’t require multiple Draft folders to accomplish that. There will always be a line where it makes more sense to split a project off though. While you can do a lot with overriding section types, and having different compile formats with different style treatments, have statistics track only the work you’re focused on, binder hoisting to isolate the list to one section of it, and all that stuff, I would say main factor would be metadata, depending on how much you use it. Keywords and labels for example could get very cluttered if you use them to specifically refer to things that are only applicable to one work. For closely related works there may often be enough overlap that it is fine though—same characters, same locations, etc.

For cases where it gets unwieldy though, it is good to know that separate projects can still work closely together. And here is a recent discussion on tips for multiple works in one project.