Oooh, I'm in your same shoes. I'm doing an internship this summer for my career, and I just received an email from my future supervisor with all of the relevant information I will need for the temporary position.
As for what you should tell HR when I originally spoke to them (I discussed with the experts and the ETL), they needed specific dates from starting to end, and they want to know why (for school or just development) so they can accurately plug your information into their system. You can always extend or shorten your LOA, but you need to call them ASAP, because typically what happens is that your end date is your date to deal with, so they'll call you to make sure everything is going well when you're about to return, but if you don't let them know that you're coming back, then the position is no longer yours. I'm not too sure what happens if you're still in a probationary period; that's a conversation you need to have with your leaders or another HR personnel on this forum that's more knowledgeable than me. If it's anything like transferring to another store, you're basically stuck at your current locale and you can't do anything unless your 90 days are up.
With me, my internship goes from May to September, but I honestly don't plan on coming back to target after it's over. Three years working at target, and 4+ in retail is long enough for me. I've grown jaded and fatigued, and there's more out there for me, but good luck with all your future endeavors. Maybe this internship will offer more opportunities for you to seize.
