wrote

I buy lots of oxygen from the welding store and the label always says
USP grade. I have never seen any other kind. The answer is on the
label tho.

That is because the oxygen itself it USP grade. One difference I forgot to
mention. Filling procedure. You can fill a welding grade bottle by making
the connection, opening the valve, and filling. When filling medical
bottles, they must be emptied, hooked to a vacuum pump and evacuated, then
filled.

Welding grade can be used in many ways by many different people. You can
hook it to a manifold along with other gasses. If the cylinder pressure
drops below what other gas on that manifold it, it can be back-fed some of
the other gas and contaminated.