Usability testing is mentioned as having taken place, but it’s not clear what the findings were, or how those findings translated into improvements to the design.
Usability testing has resulted in some clear findings, which are addressed in iterations to the design.
Usability testing has been done on the product itself, as well as with potential competitors or comparable products, to paint a more complete picture of how users would use the product. The connection between testing findings and design decisions have been made obvious.
Usability testing has been thorough and yielded deep insights into user behavior with the system, resulting in clear improvements throughout the product. The product itself leaves no doubts about its usability, as even minor issues and edge cases have been considered.