I'm looking at the ipv mini 30 watt on myvaporstore.com and I'm confused by the battery selections. They list 4 options, all 18650, which from my understanding is the size/shape of the battery, then they have gradually larger mAh ratings, which is how much power they hold. What im confused by is that 3 of them say flat, followed by 25A, 30A, and 35A. What does flat mean and what is the corresponding number?