The one in Tokyo ~20 years ago, they used plates of different colors/designs to indicate different pricing for each dish. I just stacked the plates on the side until I was ready to settle the bill, then a server came over to tally the plates and told me how much to pay. [*]
The ones I visit in the US today, all the dishes off the constantly moving conveyor belt are priced the same, and there is only one plate design. I insert empty plates into a plate return slot specific to my assigned seat that, I believe, drops the plates onto yet another conveyor belt to bring the empty plates back to the kitchen. The number of plates are automatically tallied for final billing as I insert them into the plate return slot.
Also, reflecting different food safety regulations, the ones in Japan were served on the conveyor belt without cover, whereas the ones in the US today are covered under plastic domes on the conveyor belt.
Also, in the U.S. ones that I have visited, there is yet a third conveyor belt that's only used when I order "special items" -- dishes that I would order explicitly using a tablet interface, as oppose to dishes regularly replenished onto the main conveyor belt by the kitchen without anyone ordering them explicitly.
[*] The old Tokyo system reminds me of yet another old restaurant billing system that I don't see anymore -- using differently colored plastic spoons to indicate different prices for drinks. Every drink was served with a plastic spoon. When the patrons are ready to pay, the server would tally up the cost of drinks by the differently colored spoons they see on the table.
