Chapter 1 What’s New for FortiOS 5.2.1 : System Features : Netflow v9.0 : Adding Netflow Sampling to an Interface
  
Adding Netflow Sampling to an Interface
Netflow sampling can be enabled on an interface to sample transmitted traffic (tx), received traffic (rx), or both using the CLI:
config system interface
edit <name>
set netflow-sampler {disable | tx | rx | both}
end
end