Used for: splitting items that're in a list into separate items.
This rule does the exact opposite of the "group items" rule. In this case you select a field of a item that contains a list (e.g. a list of available sizes) and create separate items for each element in that list. This way, an item with a list of e.g. 5 sizes will be transformed into 5 products that are exactly the same except on the field "size".
As you can see below, the rule is quite easy to set up. Just make sure that the "size" field contains a list of available sizes, otherwise it will not work.