What kind of detail is the 'half-moon shaped belt kkang' mentioned in the slacks description?

Belt kkang is a term used on garment production sites to refer to a belt buckle. With the addition of half-moon shape, it refers to a semicircular metal buckle with one rounded side, like the letter D.

RE RHEE's STRAIGHT WIDE LEG SLACKS TROUSERS includes this half-moon buckle belt as a component in both the BK (black) and GY (gray) options^[1][2][3][4]. You can tighten the waist fit without having to pick out a separate belt, and when you tuck your top into the pants, the gently curved buckle creates a neat finish at the waistline.

hmmm, though it is written in unfamiliar trade terminology, it is essentially the brand's thoughtful touch of picking out and providing a buckle shape that complements the pants' silhouette.