In the Document Usage object, there is a field called Channel that records the channels through which the content is used, such as Vault, Brand Portal, or External.
External is used for all the channels outside of Vault. Is there a way to identify the exact application which is using the documents through the External channel? How is the system identifying if the Channel is External?
There is currently no way to identify the exact external channel.
The system determines if the action comes from Portal or Vault. If it is neither of those, it is identified as External.
Vault Help Documentation: Document Usage Metrics