A 30F is a 30 band with an F cup — a generously full cup. The band does most of the work (around 80% of your support), so if it rides up at the back or the straps dig in, it's the band to fix, not you.
We cut 30F for fuller busts, part of our full 30D–30K range. Not sure 30F is your size? Book a personalized fitting and we'll get it sorted.