Unknown CSF (ConfigServerFirewall) MAIL RELAY parameters:
RT_RELAY
RT_AUTHRELAY
RT_POPRELAY
RT_LOCALRELAY

What is the meaning of these?

RT = Relay Tracking. "This allows you to track email that is relayed through the server."

Probable meaning:

RT_RELAY - external e-mail?
RT_AUTHRELAY - e-mail authenticated by SMTP AUTH, sent by remote e-mail clients or PHP script that are logging into my SMTP server and using it to send e-mail on their behalf
RT_POPRELAY - e-mail authenticated by POP before SMTP". Some older e-mail clients use this
RT_LOCALRELAY - e-mail sent by the PHP scripts and others that are using /usr/sbin/sendmail or /usr/sbin/exim directly
RT_LOCALHOSTRELAY - e-mail sent by the server (from command line, server crontabs maybe)

Source of many information:
https://forum.configserver.com/viewtopic.php?t=6434
https://www.knownhost.com/wiki/security/csf-lfd/notifications