Bob, is mine a 22? I think it's a 22. It looks virtually the same, but I think where Rich's has the wider space between the 3rd and 4th button, mine are spaced equally. Or maybe there's one more button. I'm trying to recall the buttons: Soak-n-Scrub, Normal, Light, Quick/Glass, Rinse/Hold, Dry Heat Off, Sani Rinse, Cancel. Yes, eight buttons. Not necessarily in that order. Rich's has seven.
Rich, your Custom 17 and this Superba 22 are quite different.
The Superba has a wash arm under both racks, adjustable pins in the upper rack, upper rack raises/tilts, lower rack has (I think) a removeable section (mine does), silverware basket is different, includes a rinse agent dispenser, has a water heating element in the sump, has a hidden rapid-advance timer, lower wattage drying element. And of course, more cycle selections. The Custom has only Full Cycle and Rinse/Hold, plus no-heat dry option.
The Custom Full Cycle (assuming it's the same as the Imperial 17A) begins with a half-fill line purge and drain. Then wash, rinse, wash, rinse, rinse, dry.
I can't say about the Superba 21, but my 22 doesn't do a purge because of the automatic water heating. Normal always heats the first wash to 140°F. It doesn't do double rinses. Wash, rinse, wash, rinse, dry. There's a 10- to 15-second fill with 1 minute recirculate before both rinses (I think before both, I know before the last) on Normal and Soak-n-Scrub. Light purges only before the final rinse, and doesn't heat the first wash. Soak-n-Scrub is wash, wash, rinse, wash, rinse, dry. It recirculates during the first fill, then pauses to heat to 140°F. Sprays for 1 minute, then pauses to reheat. Sprays for 1 minute, then drains. Refills, reheats to 140°F, wash. The rest same as Normal. Quick/Glass is fun. Begins with a prerinse, but only circulates while filling, then immediately drains. Main wash. Final rinse (no pump purge before). Dry is shortened.
Note that if your machine has the Soak-n-Scrub cycle with two leading washes ... the detergent dispenser has two covered compartments, and the dose for the first wash is simply placed in the depression between the two cups when they're closed. It immediately dumps to the bottom of the tub when the door is closed.