External Subtypes let system admins define Subtypes for documents that route somewhere other than Safari's fulfillment systems — most commonly Non-Served Documents (NSDs). An NSD is any document that doesn't qualify as a Served Document, such as junk mail, customer complaints, or anything else that doesn't require service.
Once created, External Subtypes show up in the Reclassify dropdown (see SDM - Document Viewer Actions) alongside the standard Safari Subtypes. That way, a reviewer can classify these outside documents so they can be routed and reported on automatically.

On the SDM External Subtypes page, select NEW and fill in the following:

Once saved, the Subtype appears in the Reclassify dropdown in the Document Viewer. Note that when your company subscribes to SDM, the platform automatically creates a Junk External Subtype as an NSD; you can deactivate it on the SDM External Subtypes page if you don't need it.