Whilst there are subtle differences between the Usuba and the Nakiri knife the main difference is that the Nakiri knife is double beveled - both knives are made to slash vegetables.
Usuba or thin blade knife. In Japanese 'usui' means thin and 'ha', as in hamono, means blade. In Japan you're more likely to see an usuba in the hands of a professional chef. Along with the deba and yanagiba this knife is one of the three main knives used in a Japanese commercial kitchen. The Usuba is the thinnest of the three general knife shapes. Flat edge profile and square tip. Used for push cutting, rotary cutting thin sheets and cutting thin strips from those sheets.
The Nakiri knife is the traditional vegetable knife for the professional Japanese chef. This Vegetable knife with thin & straight blade for clean cutting paper-thin slices are no problem, even with ripe tomatoes.
Like other Japanese professional knives, usuba are chisel ground, and have a bevel on the front side, and have a hollow ground on the back side. Usuba knives characteristically have a flat edge, with little or no curve, and are tall, to allow knuckle clearance when chopping on a cutting board.
Usuba literally means thin blade indicating its relative thinness compared to other knives, required for cutting through firm vegetables without cracking them. Due to the knifes height and straight edge, usuba are also used for specialized cuts such as shaving a vegetable cylinder into a thin sheet.
While the nakiri bōchō's cutting blade is sharpened from both sides, the usuba bōchō's blade is sharpened only from one side. This style edge gives better cuts and allows for the cutting of thinner slices than the style used for nakiri knives, but requires more skill to use.