I had a quick question. When using an algorithm such a "Customers who viewed this ultimately bought", if I am displaying a 5 product recommendation zone but the algorithm only has 3 products associated with the selected algorithm, how does it fill in the remainder of the recommendations? Is there a waterfall of criteria that it looks at to determine the next best matches for the remaining 2 recommendations?
As Mihnea mentioned in a specific criteria (aka algorithm) you can choose to allow "backups" (a random selection based on most popular) or have a partial fill (only send 3 instead of 5 results) or neither, in which case Target Recommendations wont return any results unless they are at least as many as requested.
However, there is also the ability to stack or sequence criteria. So if your preferred criteria only returns 3 of 5 results a second or third criteria (chosen by you) can fill the remaining slots. You can set this up in the UI (Recommendations > Criteria > Create Criteria > Create Criteria Sequence).