Backstories - What should be in them?

I have been looking into the topic of backstories but find there is a little confusion about what should or should not be in them. I have therefore set up a quick, simply survey that will only take a minute or so to complete. Your participation would be appreciated and will help fellow authors and readers. Here is the link to it.

https://bit.ly/2zBcnyk

Thank you (in anticipation of your participation).