Corral placement is based on proof of time. If you believe you will finish the marathon sub 5:00 hours, proof of time is required.
Acceptable forms of proof of time are either a photocopy of a finisher's certificate or a print out of race results. All forms of proof of time must be from a marathon, half marathon or any race 10 miles or longer, after July 1, 2004. Online registrants, please answer all questions regarding proof of time. If you register by mail please include proof of time with your entry form and payment.
If you do not provide proof of time you will be placed in the last start corral.
I did not have "proof of time" the first year when I signed up but I ran an 18 mile race that put me at about 4:30 for the marathon. At packet pick up you go to the "runner relations booth" with your race results (proof) and they will then assign you to the correct corral. IMO, this is only important if you are planning on running the marathon and want to avoid the congestion of the walkers who usually start in the last few corrals.
here is an email I got from the race director regarding this:
You will need to bring proof of your estimated finish time to the Runner Relation's booths after you pick up your race number and ChampionChip at the Marathon Expo. There, the race staff will place a sticker on your race number to allow you into a faster corral. You must have that proof with you to get this change made.