Coupons are special rewards that can be obtained for free while participating in events, obtaining regatta chests and yacht club rewards. You can use any of these single-use Coupons immediately after obtaining them, whereas a Booster is activated at the Laboratory and is available to use after you have received it or purchased it with gems in the Laboratory.
It's best to use these coupons on your highest level factory, island or train line.
After you claim or receive this type of coupon, you can't choose to use ingot(s) to upgrade any factory, island or train until you have used up all of this kind of coupon first. Also, you can't view the quantity of ingot(s) required for the upgrade you want. To save ingots in order to make more upgrades, you can take a look at the Academy of Industry page and use the coupon for the upgrade which originally requires the most ingots.
Factory Upgrade Attributes
Train Upgrade Attributes
Island Upgrade Attributes
Ways to obtain Coupons
Coupons are available during Sales and in the Yacht Club.