There are three ways to enroll a recipient into a campaign:  Enrollment Triggers, Email Lists or Manual Enrollment

Enrollment Triggers

Enrollment triggers are part of a campaign's general settings. To manage them, navigate to the Autoresponder home page.

...

• Abandon Cart – This trigger will enroll any abandoned cart with an email address into the campaign.
• Purchased Product – This trigger will enroll any customer making a purchase into the campaign. Enrollment Items can restrict enrollment.
• Auto Order Decline – This trigger will enroll any customer who has an auto order declined.
• Auto Order CC Expiration – This trigger will enroll any customer who has an auto order credit card expired.
• Auto Order Canceled – This trigger will enroll any customer who has an auto order canceled.

Enrollment Items

The enrollment items work in conjunction with the enrollment triggers. They are filters for the triggers. If you provide a list of items, then only events with those items will trigger an enrollment.
For example, consider the result if you add an item id, such as 'SOMEPDF' to the enrollment items field and check the enrollment trigger 'Purchased Product'. With that configuration, only purchases of SOMEPDF items will result in enrollment. This is useful if you wish to upsell certain items or provide item specific marketing material.

...