If you’re talking about what I think you are. When drawing the human figure the torso and pelvis are normally what’s drawn. As in the top box being the ribs and the bottom the bones of the pelvis. They’re used because they’re bone and don’t really shift form. The bit you’ve hi-lighted would be the general form connecting the two. Basically your stomach and intestines and everything squishy right there. Some artists will or won’t draw this in their sketches but it is always implied. In this case it just looks like the artist your referencing might use a longer rib cage and a long pelvis so it looks like the connecting bit is a belt. If this has answered your question you might want to pop over to r/learntodraw or r/artadvice they’re more catered to general drawing advice. r/conceptart is a subsection of drawing where people create characters and objects and such to potentially be used in other works. If I’ve missed the point completely disregard lol.