System logs display entries for each system event on the firewall.
1. Critical - Hardware failures, including high availability (HA) failover and link failures.
2. High - Serious issues, including dropped connections with external devices, such as LDAP and RADIUS servers.
3. Medium - Mid-level notifications, such as antivirus package upgrades.
4. Low - Minor severity notifications, such as user password changes.
5. Informational - Log in/log off, administrator name or password change, any configuration change, and all other events not covered by the other severity levels.
https://docs.paloaltonetworks.com/pan-os/11-0/pan-os-admin/monitoring/view-and-manage-logs/log-types-and-s