Authors and Writers use the term Backstory in different ways. Most think backstory is a history of events and causes for what happened before their story started. A narrative is often placed in the prologue to set the stage for the action that follows; However, if not done properly it can put the reader to sleep. Others add little snippets of their backstory as required to set up the characters and plot.
A better choice would be the word Backgrounder or Background Story, to differentiate the earlier events and histories of those characters, what caused their motivation, the elements and places that make up the individual Scene Elements of your story. A Backgrounder is the establishment of facts pertinent to set the stage, and reveal what happened which brought the characters to the starting point of your novel.