A Mystic Signature was a unique signature that every being with magic abilities could form and manifest. Like the Life Signature, the Mystic Signature was unique per person or even more profound and can't be duplicated.
However, forming a Mystic Signature and then manifesting it were two different things. Because despite everyone having the Mystic Signature, it exists in a disarrayed state just like magic orbits, and only appears once something forms a magic core. Simply put, the Mystic Signature was birthed by the Magic Core!
But it wasn't the only way to awaken a Mystic Signature. There was another and more difficult way that was to open the Solar Plexus!