What is the relationship between accommodations & accessible instructional materials? How are they the same? How are they different? Is one a subset of the other?

Accessible instructional materials are an accommodation. Possibly a subset under this broader category. You first decide that an accomodation is needed, then you could consider accessible instructional materials as one of the possible accomodations. The accomodation is the broader category. It could involve things in the environment such as positioning, placement in the classroom, etc. Accessible instructional materials are related to the actual presentation of the learning materials.