Collectors with a custom rule are delivered to you with other default collectors. These collectors allow you to change the data sets that are collected into the default KPIs assigned to these collectors.
The difference between these collectors and Collectors with a Custom KPI is that you don't have a possibility to create a custom KPI definition, but only define new rules for the collection of custom data sets into a default KPI. (The Input table of these collectors does not contain the field for a KPI name).