Please note, we don't generally recommend setting up more than a few manual recommendations.
When you set up a manual recommendation, we treat it as if lots of the people who buy product on the left also go on to buy the products on the right.
If someone is looking at the page for the product on the left, we'll show the products on the right as recommendations.
On the cart page, the suggestions we show depend on all the items in the basket, but if the product on the left is in the basket we're more likely to show the manual recommendations you've set up amongst the cart page recommendations.