It's too early to discern exactly what this new distinction means, but at the very least, you should be able to mail heirlooms to characters on your realm that are on a different WoW account but the same Battle.net account. It's not known whether or not you'll finally be allowed to mail heirlooms to different realms, but at least you'll be able to mail some heirlooms to that Recruit-a-Friend mule account you made. Pretty cool!
We'll let you know if Blizzard gives any official word on what's happening.
WoW Patch 4.1 is on the PTR, and WoW Insider has all the latest news for you -- from previews of the revamped Zul'Aman and Zul'Gurub to new valor point mechanics and new archaeology items.