What is the dial on the bottom for?
The dial just shows in what portion the dishwasher is running..ie prewash, wash, rinse, dry. It is not necessary to turn the dial for under normal circumstances.
The 15 series boasted "Instant Start" which meant that once a button is pressed, the motor would come on and it would start filling immediately. One could advance the timer manually to the "dry" portion if you wanted to warm your plates since the 15 series didn't have a special button for that.
The Superba model had a rapid advance timer so after the Rinse and Hold and Utility Utensil cycle, the rapid timer would advance it to the proper place within a minute or so rather than waiting for the regular timer to advance step by step.
The Imperial and Custom models weren't equipped with the rapid advance timer, so one could just turn the timer to "OFF" after the Rinse and Hold if they didn't want to wait for the timer to do it.