you're wright
when a user uploads file he sets his download credit required for his file(1-5)
you have 30 download credit a day
lets say user sets his download credit to 3,when you upload his file you lose 3 downlod credit and you now have 27 download credits
when you lose all of your download credits you have to waite one day so you can gain 30 download credit.
you can only download 6 files who require 5 download credits,or you can download 30 files who require 1 download credit
i hope you'll understand it better now