Collectors with a custom KPI are delivered to you with other default collectors. Their main feature is that these collectors allow you to create your custom KPI and assign it to these collectors. The Input table of these collector contains the field for a KPI name and the structure of an Input table is different for each collector with a custom KPI.
Apart from that, are these collectors in their functionality the same as the Collectors with a custom rule. They allow you to change a rule or add a new rule for the default KPI and thus influence the resulting value of a default KPI.