The news broke this morning: the Des Moines Register will release its first Iowa ballot test of the 2012 Republican presidential field on Saturday night. (And, yes, that means that the Fix officially has plans on Saturday night.)

The Register poll, conducted by the esteemed J. Ann Selzer, is widely regarded as the benchmark for where things stand in the Hawkeye State. A strong showing can help a candidate raise money and build momentum, a poor one can have the opposite effect. In short: the Register poll matters.

So, what will the first 2012 poll — due out at 10 p.m. eastern time on Saturday — tell us about the field? And what won’t it tell us?

Answers below.

WHAT IT TELLS US

* Setting the baseline: This is the poll that will be used to judge momentum (or lack thereof) in the Iowa campaign to come. Every time the Iowa caucus race gets written about, the first Register poll will be cited and used as a way to judge whether progress is being made. It’s the expectation setter. For someone like former Minnesota governor Tim Pawlenty — for whom a win in the Iowa caucuses is close to a necessity — the ideal would be a competitive second or third place in this first survey. That would keep expectations reasonable and allow him room to grow and claim genuine momentum going forward.

* Romney’s burden: It’s a near certainty that former Massachusetts governor Mitt Romney will be at the top of the Register poll. He spent millions in the state in 2007 and 2008 and, without former Arkansas governor Mike Huckabee, the caucus winner last time around, in the 2012 race, Romney is the best-known GOP commodity in the state. Assuming Romney is in the lead, it will re-start a debate over whether he can legitimately downplay the Iowa caucuses — as his campaign seems set to do — and instead focus much of his time and energy on the New Hampshire primary. It’s hard to imagine Romney rethinking his strategy — he has already announced he is skipping the Ames Straw Poll in August — but a strong first place in the Register poll would put him in a bit of an awkward place.

* Bachmann bounce?: The Register poll comes at a perfect time for the Minnesota Congresswoman. She is coming off of a widely-praised performance at last week’s New Hampshire debate and is, without question, the buzziest candidate in the field at the moment. Seeking to capi­tal­ize on her newfound star status, Bachmann is set to formally kickoff her campaign for president in Waterloo, Iowa on Monday. If the Register poll shows Bachmann in second or third — a real possibility — it would be a major momentum-booster on the eve of her announcement.

WHAT IT DOESN’T

* A (still) unsettled field: While the Republican presidential field has come into far sharper focus over the past month, there are still several unknowns that make polling the race — in Iowa or anywhere else — difficult. If either Texas Gov. Rick Perry or former Alaska governor Sarah Palin decides to run, the race in Iowa could well look fundamentally different than it does today. It’s not clear whether the Register poll will include either or both Perry and Palin (or offer a separate ballot test with them left out) but remember that most smart GOP observers believe the final field is not yet settled.

* Moment in time, not a predictor:The biggest mistake people make with any poll is to assume that it’s meant to predict who will eventually win. Campaigns and candidates matter and, particularly in a race that will get the amount of media attention of this caucus race, the state of play will shift multiple times between now and February 2012. Look back at the first caucus poll the Register did — in May 2007 — in the 2008 race. Former North Carolina senator John Edwards led the Democratic field with 27 percent while then Sens. Hillary Rodham Clinton (N.Y.) and Barack Obama (Ill.) took 20 percent and 19 percent respectively. The numbers in the GOP field from that time are even more illuminating about the changeability of the caucuses: Romney led with 29 percent followed by former New York City Mayor Rudy Giuliani and Arizona Sen. John McCain who each took 16 percent. Huckabee didn’t even break double digits.