When you go to create a video via the script editor or the Create New workflow, you're able to have assets and music added automatically from Storykit's stock libraries and music libraries. This feature is extra useful if:
you don't have a project library set up
you're a user who doesn't have asset libraries, or you don't have your assets libraries well organized
This feature is set by default so, if you create a new script you already have it toggled on. If you don't want it you need to turn it off.
When Include Music is on and the video theme's rule set has Music Settings, those settings decide which track is added. If you turn Include Music off, no music is added, even if the rule set has Music Settings. See How Do I Choose the Music for a Rule Set?