DRIVE A controls the amount of Class A power amp output stage distortion (like Vox AC30®-style amp), rich in even harmonics. The amount of harmonics is dynamically controlled by the input level and their ratio by how hard you pick.
DRIVE A/B controls the amount of Class A/B power amp output stage distortion (like Fender®/Marshall®-style amps), rich in odd harmonics.
The signal path of Drive A runs into the signal path of Drive A/B. This results in an electronic multiplication, rather than addition, of the amount of distortion to achieve a cascading effect.