How products are sorted in the related block when the Source Type is set to “Bought Together”?

How products are sorted in the related block when the Source Type is set to “Bought Together”?

With Source Type set to Bought Together in the related rule, products will sorted based on their frequency of being purchased together, i.e. by the number of purchases. For instance, if customers buy product X alongside product Y more frequently than with any other products, product Y will appear in the first place.