All A/T cars nowadays have an OD function
OD allows you to go to 4th gear and the ECU will rely heavily on the pressure you apply on the gas pedal to know what gear to switch to. If the OD is turned off, you are limited to 3rd gear. The usual practice is to always turn on OD especially on fast roads so the the engine won't rev too much when going beyond 100km/h. Turn off OD when going uphill or go to the lower gears.