B stands for levels of solid achievement and kindness. While hard work and long hours can be beneficial, they don't always guarantee success. Kindness is about the combined results, not just effort. The potential for improvement can be quite simple.
A B rating indicates that the restaurant is likely to be doing everything correctly 80% of the time. Exceeding that extra 20% will earn you an A grade. Any grade is a sign that it's safe to eat there. If it wasn't up to standard, the place would be shut down.
Taco trainers don't need any qualifications.