The F.A.T. is the Fast Acting Thermistor upgrade for your new air intake system. The thermistor measures incoming air temperatures and sends it to the computer to perform calculations for fueling and timing. The faster the sensor can accurately signal a change in temperature the better especially when seconds count.
The genuine Omega thermistor has a much shorter time constant than the factory sensor plus the extended sensor mounting places it further into the air flow and away from its heat soaked body.
The LS1 just requires mounting and connecting it to the factory plug. LS2s require a Caspers electronics break out harness
and is then plug and play.