An IEP is the more involved version of a 504 Plan. An IEP is part of the Americans with Disabilities Act which governs both schools and workplaces. IEPs (individual education plan) is exclusive to school children or students in grades K-12. Under an IEP students are eligible for more services including but not limited to assisted technology, resource classrooms, one-on-one aids, etc.
Once students who have an IEP graduate from high school, they are only covered under a 504 Plan if they continue onto post-secondary education such as college or trade school. At that time, students with a 504 Plan in a post-secondary setting would be eligible for additional accommodations such as extended time on tests, testing in a private room, use of calculator, priority housing, notetaking assistant, etc.
Often times when students have a 504 plan in K-12, it is temporary and will only stay with them for a few years at most. 504 Plans are not limited to disabilities; rather a student who gets into an auto accident and is injured would be eligible for a 504 plan, just as an example.
