You might not see these as much as RTM or even OGF, but they're the two that are currently at the forefront of the team's minds. The devs are chasing ZBB - the Zero Bug Bounce. I've also heard it defined as the "Zero Bug Balance" or "Zero Bug Backlog", both of which actually make more sense than the common definition. What is ZBB? It's when we've cleared out all of the "old" bugs - the ones that have been sitting around waiting to get fixed. We "bounce" when ALL active bugs are "new"; usually, this means, open < 48 hours (sometimes it's a week, or 24 hours, but 48 seems to work best).
In theory, once we reach this point, we should stay "in" ZBB - as in, you keep your fix rate at least as high as your incoming rate. With a coding milestone, that never lasts - once the next milestone starts, devs cut back on fixing and start coding new bugs (I mean, features) again. So, that's where the "bounce" comes from - you get down to ZBB, and then start up again (on a graph of active bugs over time, the line "bounces" off of the X axis). And of course, even in our run up to RTM, if us testers get the incoming rate too far ahead of the fix rate, then we may "bounce" and ZBB multiple times. But, the goal is to hit zero balance once, and then stay within the 48 hour window.
So, what's ZRB? Zero Resolve Bounce. This is basically QA's version of the same 'progress indicator', when we've closed or reactivated all of the 'old' resolved bugs. Again, during coding milestones, we tend to hit it and then bounce again, anytime fix rates get too far ahead of our close rate. Since devs are pushing hard to hit ZBB, we're naturally having to similarly "step up" to hit ZRB - this time around, we get an extra week after the target ZBB to hit ZRB.
After that, the goal is to keep "clean" - no bugs take longer than 48 hours to get addressed, and no bug takes longer than 48 hours to be closed (or reactivated) once resolved. As the bar goes up, this gets easier - after all, we should be finding fewer and fewer bugs - and more importantly, less and less bugs will be fixed; they'll be postponed, and those take less time to resolve and even less to close.
On a semi-related note, all of these TLAs are not 'technically' acronyms at all, but rather initialisms - we say "Z-R-B", not "zerb" or something wacky like that (though "zerb" suddenly has a nice ring to it).
We've gone from M0 (before I started at MS), through M1, M2, M3 (3.1, 3.2, 3.3), CC (code complete), Beta1, Beta 2, Beta 3. We're coming up fast on our (hopefully) final ZBB and ZRB - after that, there'll probably be one F.inal T.est P.ass, and we'll be out of TLAs to talk about until the one that counts: RTM!!
Time marches on...