A 34FF is a 34 band with an FF 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 34FF for fuller busts, part of our full 34D–34K range. Not sure 34FF is your size? Book a personalized fitting and we'll get it sorted.