What is the bar below the XP bar when I'm in a party?

I've noticed that when I'm in a party, a new bar appears below the XP bar. Hovering over this bar does not explain what it is.

What is this bar, and when does it appear?


The same bar appeared to me from time to time and I could not figure out when exactly did it show.

After finishing Act 1 and unlocking my Legacy, the bar is displayed all the time and it shows my Legacy Experience.

For now, most of the Legacy features (whatever they are supposed to be) are not implemented, so it's actually useless.