"On Board Diagnostic" emissions control ."II" is for the newest revamped version that came out in Preludes, and most cars, in 96.
The system uses multiple sensors to monitor several areas of the engine, fuel system, exhaust, ignition, to maintain pollution control. The OBDI was less restritive allowing for better gains with modifications to the above mentioned areas. OBDII is much more restrictive causing the ECU to override a lot of the initial gains in the name of lower emissions output when you change or "modify" a certain area, such as the intake. The system is trying to maintain a certain level of emissions output.
I hope this helps.