It's called armoury bonus and it only applies to combat classes (DoW/DoM)
Not at lv 50 either, just anything past lv 10 and it applies.

How it works is, if you are playing a class that is NOT your highest lvl combat class, you get a 50% exp bonus. Difference doesn't even have to be a full level.

Say your highest level class is lv 25 MRD with 90% of the exp bar filled.

Then when you play any other combat class below that level, you'll get a 50% boost until you get to level 25, 90% exp bar.