The command to enable this function is highlighted below.
fortinet has a posted KB on what fields are deleted within the brief-log-format.
You can inspect the log data output. Here's a tcpdump output of brief & normal. The brief mode will contain less payment and draw less amount of bandwidth in heavy log/per-sec environments.
NSE ( network security expert) and Route/Switching Engineer
kfelix -----a----t---- socpuppets ---dot---com
=( @ @ )=