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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 with wish to upsell certain items or provide item specific marketing material.
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