devtrackers.gg
from the custom texture
External link →from the custom texture
External link →I don't know, but left and right sides seems likely, you know, the sides of the torso under the arms
This is correct. LS stands for "Left Side", and RS stands for "Right Side". LS covers the left side of the torso, while RS covers the right side of the torso.
These areas are square-ish in proportions. Someone might have expected it to actually be rectangular, but the areas directly above LS and RS are where the corresponding arms connect to the torso.