Well, it might mean that there is an aspect of a person that is shaped by true events. For example, someone who overcame a difficult situation in real life and as a result, developed a new part of their personality, like a more resilient self. It could also refer to a character in a story that is modeled after a real person's experience, representing an alternate version of that person.
'Another self based on true story' might be about how our real - life experiences can create a different side of us. Think of a person who has been through a war. Their true experiences there might give birth to a more cautious, survival - oriented self. This new self is still them but is a distinct part that is rooted in the true story of their war experience. It could also be in the context of self - exploration, where we look at our real - life stories to understand and define these other selves within us.
It could imply a version of oneself that is inspired by real - life events. Maybe it's about a transformation or discovery within a person that has a basis in real experiences.