I work in retail, so I can only speak in general terms, but generally...
Juniors sizes are always odd numbers. They usually seem to stop at about a 15, but I do see 17s on occasion. Some stores might have larger junior's sizes available. A junior's cut is definitely narrower and slimmer, somewhat smaller, and sometimes shorter than a misses garment even if the size numbers are close.
Misses sizes are even numbers. In pants, they usually stop at 16, but I sometimes see 18s in that department depending on the brand. Tops usually stop at an XL. The garment cuts are are average.
Women's i.e Plus sizes are also even numbers. They do usually have a W after the size if the garment is pants (18w/20w, etc.) I usually see Women's pants start at a 16W or 18W on up. For tops, the smallest women's size I sometimes see is a 0x/2x/3x on up. Women's size garments are usually markedly wider and more generously cut, and seem to have roomier sleeves for example when it comes to tops. The necklines might even be a big lower to accommodate a larger figure silhouette.
We have extended sizes online that we don't stock in the stores, like XXL tops for junior's and misses. XXL indicates a junior's or misses extended size. Women's tops at our store is always "2XL" i.e they have a number in them.
OVERALL WEIGHT LOSS