Diagnose commands guide

Fortinet training courses include important details about troubleshooting and analyzing debugs.
Another source is the community articles, which share configurations and possible issues verified through logs for specific topics. You can also refer to the CLI Reference guide which provides a summary of all commands and their details.

You won’t find a full “decode everything” guide for every diagnose output because it doesn’t exist publicly. The best sources are the official Fortinet training, community articles, and the CLI Reference, which explain the most useful diagnose commands for real troubleshooting.

Hello,

For ALL of the possible outputs, there is no official document. Usually the admin guide or the study guide of the target exam will show a lot of debugs for working scenarios, that you can compare to a non working one you are trying to troubleshoot.
If you want to get pro with a few of them the only way is testing in a LAB enviroment, breaking connections on purposes and running debugs on the meantime.
A good resource to search through for extra information is our community page and our doc page. 
https://docs.fortinet.com/
https://community.fortinet.com/t5/FortiGate/tkb-p/TKB20