It’s an 100% EXP increase from levels 1-79, but drops to 50% from 80-90. For example, if you have a level 86 Samurai, the bonus will apply to any other Job you have that is level 85 or below. This buff applies to any job below the highest level of your main one. Heat Of Battle I and II can be bought from Grand Companies, while III needs to be crafted on an aetherial wheel in player housing. There are three tiers (I, II, III) that give you a 5%, 10% or 15% EXP boost respectively in battle.

Luckily it’s a very commonly used buff by most Free Companies and you’ll easily find people recruiting in Limsa Lominsa. This buff is only available if you join a Free Company and they use an action to put it up for everyone.