Purposeful Harbinger:
Aura Buffs from Skills have 8% increased Effect on you for each Herald affecting you, up to 40%.
If you gain Aura's from other character's this acts as a multiplier. For the user itself it is just regular Aura effect and additive with all the other Aura effect.
It would be easier to see/calculate your breakpoints on Aura effect if this was added to the Aura Effect overview in the Calcs tab.

Thanks!