A conversion harness is used to switch a CARs ECU to OBD1. or from OBD0 to OBD1. in "honda" world, the standard for ECU's has been OBD1. THey have Onboard Roms that are able to be hacked and modified, unlike OBD2 where there is no ROM on the ECU board, there is nothing to "chip" so to speak on an OBD2 ecu.

90% of the systems available for hondas aer built around the OBD1 code (hondata, Uberdata, Crome, etc)

The only option for OBD2 cars is FULL STANDALONE computers that eliminate the factory ECU, like AEM EMS.

The conversion harness will convert a obd2 car to obd1, then you run an OBd1 ecu and you have a much more user friendly ecu and car to tune and drive. However, when emissions time comes around (if it applies to you) depending on the year of your car determines what test they do. if you have a OBd2 car, they must scan the dataport and scan the ECU for CEL. if your OBD2 car is converted to OBD1, it wont pass, cause the obd1 ECU doesnt use the same protocol. you have to switch it back to obd2 (which is as simple as plugging the OBD2 ecu in.) however, throw a CEL and you will fail.