When soft, still plyable anal dischanrges rub so close to the offenders' undergarments that a fecal stain is left behind. May also occur when the issue is "forced." I.E. Butt picking, massaging of anal irritations, or sloppy wiping techniques. See also "streaks."
Dammit Donnie, the line for the port-o-let was so long I had to rush my post crap rectal cleansing techniques. Now I have smelly 7-1/4 inch skidmarks as a result!