What kind of waist structure does the 'obi line' written in the pants description refer to?

It is the structure you encounter at the very top waistline when wearing pants. In garment manufacturing sites, 'obi' is an industry term referring to the waist band, or waistband, that wraps around the waist of pants or skirts. Rather than finishing the waist seamlessly in one piece with the body panel, a separate fabric band is wrapped and attached, creating a distinct horizontal seam line on the exterior.

RE RHEE wrapped this obi line around the waistband of the wide-leg trousers to distinctly bring out the outline of the band shape on the outside^[1]. Unlike methods where seam allowances are folded inside the waist to hide them smoothly, an overlaid band line resembling a worn belt is neatly revealed on the exterior.

When tucking a shirt or T-shirt into the pants, this separated waistband clearly defines the boundary between the top and bottom. Even without wearing a belt separately, the waist circumference finish does not feel empty and falls neatly.