@RiderFan
Are you sure that the black line over her ass is not a crotch seam (extending from the gusset)? To me, it looks like one :3.

The crotch seam tag is meant to be used when there is a seam covering the crotch.

Crotch:

1. the part of the human body between the legs where they join the torso.
2. the part of a garment that passes between the legs.

The whole area between the legs is referred to as the crotch, thus the middle panel of panties are referred to as the crotch panel, or sometimes gusset. The crotch seams are the sewing lines that join the crotch panel to the rest of the panties. There are usually two such lines, one in the front and one in the back, although there can be less.

If you were to tag this as crotch seam then every image with gueset as the tag would also have to include the crotch seam tag. Crotch seam posts that offer complete clarity on what the tag is meant to define:
post #2675493, post #2455531, and post #2562523.