The Approved Header Field List for Gmail specifies the header field criteria for email messages in Gmail to bypass policy scanning when a message matches any of the criteria.
If you want certain email messages to skip scanning by all policies, for example, to do a phishing simulation campaign via email, you can create a global approved list so messages that match the header field criteria will not be scanned by all enabled Advanced Threat Protection and Data Loss Prevention policies for Gmail and will be delivered to the intended recipients.