What must accompany the subject line marking of a classified email?

The subject line marking of a classified email must be accompanied by a portion line. This is crucial because the portion line indicates how the information within the email is classified and provides guidance on its handling. It helps recipients understand the classification level of the overall content and indicates whether certain parts of the email may be unclassified, classified at a lower level, or remain fully classified. Proper use of the portion line ensures that classified information is appropriately accessed, shared, and protected, thus maintaining the integrity of the information and compliance with security protocols.