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