Chapter 15 Unified Threat Management for FortiOS 5.0 : Web filter : Inspections Modes
  
Inspections Modes
Proxy
Proxy-based inspection involves buffering the traffic and examining it as a whole before determining an action. The process of having the whole of the data to analyze allow this process to include more points of data to analyze than the flow-based or DNS methods.
The advantage of a proxy-based method is that the inspection can be more thorough than the other methods, resulting in fewer false positive or negative results in the analysis of the data.