After mapping a segment to an external destination, the user notices in the dataflow run...

These are two possible reasons why some identities have been excluded after mapping a segment to an external destination. Some identities were not recognized by the destination means that there was no matching identifier between AEP and the destination system. For example, if AEP sends email addresses as identifiers but the destination system expects phone numbers as identifiers, then there will be no match and those identities will be excluded. Some identities were not meeting consent policies means that there was a violation of data usage governance rules based on data labels and marketing actions. For example, if AEP sends PII data as part of a segment but the destination system is not authorized to use PII data for personalization purposes, then those identities will be excluded. References: : Identity Mapping Overview : Data Usage Governance Overview