There is the IEP and then there is the 504 plan.

From what I recall the IEP was more for a visible disability or maybe not so visible but could or does effect the learning of the child.
The 504 would be for any condition that needs awareness by the school for medical reasons but does not have a day to day impact on the childs learning.

When I did a "GoodSearch" I found MANY answers. The site below had a good one...

http://specialchildren.about.com/od/504s/f/504faq2.htm --

"A 504 plan, which falls under the Americans with Disabilities Act, is an exercise in civil rights, an attempt to remove barriers and allow students with disabilities to participate freely. An IEP, which falls under the Individuals with Disabilities Education Act, is much more concerned with actually providing educational services. Students eligible for an IEP, or Individualized Education Plan, represent a small subset of all students with disabilities. They generally require more than a level playing field -- they require significant remediation and assistance, and are more likely to work on their own level at their own pace even in an inclusive classroom. Only certain classifications of disability are eligible for an IEP, and students who do not meet those classifications but still require some assistance to be able to participate fully in school would be candidates for a 504 plan."

When my son was in the public school (He is home schooled now) he had a 504 plan in place. He did not need anything special educationally.
(ummm, actually he DID need more academically but not because of CAH ...because he is advanced vs behind and the school has NOTHING for the kids that need more academically! Which is why he is now homeschooled.) Anyway, say the 3rd grade class is planning a field trip to the museum. This will be an all day event. The school wants to keep cost down so they do not intend to send a nurse on the trip. The school system also does not, for legal reasons, allow a teacher to be responsible for the solu-cortef emergency shot or the regular cortef medication my child would need in the afternoon. I am not able to go along because of work whatever. They could say to me that my child can not participate in the class field trip.
Unless my child had the 504 in place i which case the school would be obligated to do whatever it needed to do to make sure that my child is not excluded. If they have to send a nurse along then they have to send a nurse along.

When my son first started school I did not feel like I needed a 504. The school was doing everything that a 504 would require anyway. Well, the following year the entire top staff AND the nurse were gone with new ones in place. The new staff had no intention of following the unofficial plan from before...until I forced them to be fair by setting up a 504.