Ume vinegar (not a true vinegar) is a byproduct of umeboshi (pickled plums). When making umeboshi, the salt extracts juice, which the ume then soak in for about two weeks. This salty, sour liquid is marketed as umezu. It has a tart, salty flavor, used in salads, or to add some umami flavors to vegan dishes.
Ume vinegar makes a good substitute to fish sauce in recipes.