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Not all storylines have a storysource. Some stories might only partly have a storysource, e.g. they're loosely based on a story but with heavy deviations (more than is necessary for a storyline). Some storylines have more than one storysource, e.g. they are based on multiple stories at once. It's basically as varied as headmates in systems having sources.
For example, let's say an abuser shaped a system based on Lord of the Rings. The abuser wants their victim not to feel like they have any free will, and that it's very important their victim destroy any desire for control they have within themselves.
They might tell a story or make up a version of Lord of the Rings where, instead of the Ring, Frodo and Sam are taking another object to Mordor to be destroyed - an item that, to the victim, personally represents free will, or that the abuser teaches them to associate with free will (e.g. a key).
After familiarizing the victim with this alternate version of Lord of the Rings (perhaps via a roleplay or an AU), the abuser will then influence them to split fictives of Frodo and Sam (perhaps via roleplay, or perhaps via sending the victim fan art of the characters when the victim is dissociating/having headaches/other splitting symptoms).
Lord of the Rings is the storysource that was used, but the story in which Frodo takes something different to Mordor is the storyline that was used to shape this person.
While a storysource is something specifically existing in the outerworld, a storyline exists both in the outerworld in the form of the shaper telling it to the victim, and it also exists in the innerworld or in the victim's headmates' timelines by way of the abuser putting it there.
A storyline will typically deviate from what is in the storysource in some way. You can also have a storyline that has multiple storysources, e.g. a crossover between different media, something based on both a book and a song, etc.
Because anything can be used as a storysource, that includes real life and music. This includes events that are known from history as well as known from the personal lives of the people involved in the situation. Lyrics of songs are likely to be used as a story source if music is used in the shaping.