How the sizing works here

Sizes here are letters because that is what the maker prints. The same letter is not the same measurement across every line: a boxy tee and a bodycon dress use the same word for two different fits.

The safest move is to measure a garment you already wear and compare it with the numbers on the product page you are looking at. Where the maker gives measurements, they are on the page. Where the maker gives nothing, this site says nothing rather than filling the gap.

If you are between two letters on a fitted cut, the larger one is the safer choice. On an oversized cut the smaller one usually still reads as oversized.

Read that table as availability, not as fit advice. A department with a hundred lines in medium gives you a hundred chances. One with three gives you three.

One-size sits in its own column because it is not a size at all. It is the maker declining to cut a ladder, and it fits the way a scarf fits.