How the Veeva External Id field is populated on the Multichannel Routing records?
The Veeva External Id field is the concatenation of the following fields:
All the fields above are manually populated except for the Multichannel_Route_vod.Language_vod. This is determined by the referenced Email Template's language which is indicated on the Multichannel_Route_vod.Approved_Document_vod__r.Language_vod field.
CRM Help Documentation: Sending Consent Capture Receipts with Approved Email