Coupon
A coupon gives a percentage or fixed discount; you can set a usage limit (100 by default) and an expiry date. The code works for all your events or just one.
Seller
Add a seller name to a code and the tickets and revenue sold with it are ranked in the panel. A discount is optional: a code without a discount just tracks sales.
Link
Every code has a ?c= link: buyers who arrive through it get the code applied automatically. An event-specific code can also unlock hidden ticket types.
Misuse
Code attempts are rate-limited. Pick codes that are hard to guess and switch off campaigns that have ended.