Bungie.next is preparation.

by kapowaz, Saturday, March 09, 2013, 09:58 (4375 days ago) @ Xenos

Sorry, didn't mean to imply you'd get logged out of XBL/PSN when you get logged out of Bungie.next. I meant you could get auto-logged out of XBL/PSN in the browser due to time out with out being logged out of Bungie.net, so the stats won't update or anything.

The way cookie-based authentication works means that a cookie is associated with a domain name — e.g. xbox.com, playstation.com or bungie.net. When you visit a domain other than the domain a cookie was created for, that website cannot request information stored in that cookie — only cookies it creates itself (for this reason some larger sites create cookies at the root domain and then share this information across all sub-sites; say bungie.org could have a cookie used by destiny.bungie.org and halo.bungie.org both).

As far as ‘logged in’ state goes, really all that's happening is the site is checking if you currently have a valid cookie and if not, you're asked to log in again. That cookie existing (or not) has no impact on cookies that may exist for third-parties, and so you being (automatically) logged out of bungie.net would have no impact on third-party login status.