Filtering rules within recommendation strategies not having enough items to show

This question comes up a lot, so I wanted to clarify the behaviour when there aren't enough items to show in all slots when using filtering rules within product recommendations strategies.

• When using pin item - you are forcing the products you select to be presented in the specific slots
• When using only include/exclude - this will take the strategy results, and filter out of it products that match that rule (i.e most popular algorithm with rule to only include items with price over 100, will filter items over 100 from the most popular list). 
• In both cases - if there aren’t enough items to fill in all slots, the recommendations slots will be empty