How does the carwash know which car ordered which service?

Question

When I’m in a multiple lane carwash and we all have to merge into line, how does the carwash differentiate the cars and their ordered service?

In: 2

Answer 1

All the ones I go to either write something on the windshield in washable ink, or the automated ones base it solely on order in which it was purchased.

Answer 2

I believe the automated ones use license plate readers to match purchase with settings in car wash.