No repeat (in days) means that a product won't be recommended more than once during the specified number of days. The No repeat (in days) counter resets after the specified time and starts counting afresh for a product as soon as it gets recommended.
- Purchases – look back is one year
- Abandoned baskets – look back is three months.
By using this option you are ensuring that the products in the email will not be repeatedly shown for the same user for X number of days.
For example, if you choose 2 in the drop-down menu for "No repeat (in days)", and the same user opens the same email or emails that use the Mail campaign on two consecutive days, the system will show a different set of recommended products.
You can use this feature, if you wish to show a variety of products to a user instead of the same ones upon each open.