Jules Csillag, learning specialist & speech-language pathologist
As a point of context, IEP's exist due to the Individuals with Disabilities Education Act (IDEA) and they apply to students ages 3-21 in schools; 504 plans are a product of the Rehabilitation Act of 1973, and they apply to any agency, school, or institution that receives federal funding. They both exist to allow individuals with disabilities access to schools and other public services.
On a practical level: 504 plans are typically created if a child does not meet the criteria for an IEP, which are often stringent: your child must have one of the 13 disabilities listed on the IDEA and "as a result of the disability, the child needs special education to make progress in school in order to benefit from the general education program." For students who do not meet both of those criteria, a 504 plan may be appropriate. For example, A student with 504 plan may have ADHD and require frequent breaks during testing situations, but no other accommodations, modifications, or special education services.