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