It is an optional rule. Card numbers divisible by 10 can be used to move the circus to the lowest payoff city on the card for $20. Should be in the back of your rulebook.
We play a variant to the variant. Each person looks at the number of the largest payout they have on their initial contracts. Any card they get with the last digit matching that number is a circus for them. So if my largest payout is on card number 47, any cards that I draw that end in 7 are a circus just for me. This is public information, so we can rub it in when we draw a card that someone else has for a circus.