You will earn affiliate if someone buys the specific service you link to. You can use an affiliate link for any service, however the affiliate link is only going to work for the service you use it for, so, if you give someone an affiliate link to buy a service, but they instead buy someone else's, you will not earn an affiliate commission.
You can also promote Legiit as a whole by going to Menu > Settings > Affiliate Settings and copying your link.
When someone clicks your affiliate link they will be cookied to you for 7 days and you will get 15% commission on their purchase of that specific service, except services that have affiliate turned off. Premium Sellers have the option to turn off their affiliate settings.