Surplus is the same as Bonus Xp. Surplus is just more strict.
Example.
In WoW completing a Challenge while Rested Nets you 500 XP, While completing it unrest earns you 250 XP.
In FFXIV completing a challenge while normal nets you 500 XP, while completing it fatigued earns you 250 xp.
The systems the same, nomenclature is different. The major difference is though that with XIV the deeper into fatigue you go the less XP you get per completed challenge.