There are five log types that PAN-OS can generate: traffic, threat, host information profile (HIP) match, config, and system. All are formatted as comma-separated value (CSV) strings. This document describes the fields for each log type. The fields flagged as FUTURE_USE do not currently have predictable, useful information in them.
The syslog field descriptions in this document is now integrated into the on-device help system. Navigate to Device > Server Profiles > Syslog and click the Help icon, or search help for "Custom Syslog Field Descriptions".