How does ServiceNow match inbound email to existing records?

https://developer.servicenow.com/dev.do#!/learn/courses/tokyo/app_store_learnv2_flowdesigner_tokyo_flow_designer/app_store_learnv2_flowdesigner_tokyo_notifications_in_flow_designer/app_store_learnv2_flowdesigner_tokyo_inbound_email_and_flows

"By default, the system generates a watermark label at the bottom of each notification email to allow matching incoming email to existing records." https://docs.servicenow.com/bundle/tokyo-servicenow-platform/page/administer/notification/concept/c_WorkingWithWatermarks.html