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In my current degree (bachelor of arts, creative writing), we were told there are 4 things for a backstory:
1) childhood trauma/events
2) societal expectations
3) society in which character lived/grew up
4) relationships with others (family and otherwise).
Having said that, everything in the backstory needs to be relevant to the character in the story as writtemn and the plot constructed. Why say a character had a dog called 'Chips' when that makes no difference to the story you've written? But if you say that the character was beaten by his mother and that is why he is afraid of women in the story, that has relevance.
Does that make sense?
Anyway, I am not sure why you were told childhood does not lead to backstory.