A 28J is a 28 band with a J cup — a specialist fuller-bust 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 28J for fuller busts, part of our full 28D–28K range. Not sure 28J is your size? Book a personalized fitting and we'll get it sorted.