One of those things that was never mentioned nor explained in any of the films but retconned into the Saga by later reference materials that were approved by Lucas and his people.
Although not so much on the "Lucas and" part. I gather that Lucas paid very little attention to the tie-ins. He had people he paid to do that for him.
I wonder if they'll set this late enough in the timeline that we can get Leia as our Ahsoka type teen/child character?
I'd like to see that. Maybe if she were about 12 to 14, still a child but old enough to be fairly knowledgeable, to take on some responsibilities, and maybe have some chaste cartoon-suitable romance here and there. And at the same time there could be exploration of Luke's formative experiences on Tatooine with Biggs and the gang.