I have the Legacy version, 6-82285.
Press and hold AUX2, this will bring up the additional lighting control menu. You can turn on/off Mars light, class lights, ground lights, etc. depending on the way your locomotive is equipped. If you have Lionel’s Wi-Fi and LCS program, you can configure your CAB2 buttons to light functions you might use.
On my version, the class lights are red and used only a rear markers. They turn on and off depending on the direction of the locomotive.