The ‘B’ treats are the regular treats that are used for most of the rewards. The “A” treats are a higher value reward, like real chicken or cheese.
The “A” treats are reserved for especially good performance. This allows the trainer to reward all acceptable performance, yet still reserving some special treats for perfect execution.
Being able to reward the dog for all good performances yet keeping the dog hoping for the higher value reward is a good way to enhance performance and build drive.