Proxy/flow based inspection mechanism ?

[usmansa1]

Hi everyone, 

I was reading proxy based and flow based inspection. When i read the proxy based inspection mechanism it explains that the Fortigate deeply inspect the packet and flow based just check the pattern and packet is not decrypted in both cases unless we choose the SSL decryption profile.

Now if the packet is not decrypted then in that case the only thing that can be checked is the certificate or packet headers or may be some initial TCP packets. In this case, what is the purpose of having two different mechanism proxy based and flow based if only thing you can check is headers and content is all encrypted ? even the antivirus cant check all the attachments because every packet is encrypted ? so how does this work ?