When a rule is being created there is the option to select from 4 algorithms
The two most common that will be used for a rule set up are Default and Handpicked
"Handpicked" - This is used when you are merchandising with a specific product (Refcode) or product set (Product Set). As you are selecting the products by hand. The handpicked algorithm should be used.
"Default" - This is used in all other rule set ups which are not hand picked. If you are setting up a rule which is not using Refcode or Product Set please use the Default algorithm.