Designing External Systems
As described under What Type Of Workflow Process Is Created?, every notification upload staging (NUS) record contains an external system. An external system is the unique identifier of the sender of the notification.
To "design" your external systems, document the external id's (e.g., DUNS number) of your service providers. The following table contains a sample:
External System
Description
102910
Energy Supplier, Inc.
392191
Cheap Power, Inc.
The above example would result in 2 external systems - 102910 and 392191.
Note: 
It's obvious, but worth stressing, that the External System should be the exact number referenced on notifications sent by a sender.