: Steam sends an email for every purchase with a unique "Confirmation Number" (e.g., 1234567890123456789 ).
In the early 2000s, PopCap Games used an "Order Number" or "Registration Code" system to verify purchases. When you bought the game, you would receive a unique alphanumeric string via email. Entering this number into the game's activation screen converted the "Trial" version into the "Full" version. Where to Find Your Order Number Zuma Deluxe 1.0 Order Number