Complex sentence: The region is not clearly defined; its exact boundaries vary by source but fall mostly within the Choctaw Nation of Oklahoma's tribal jurisdictional area, and include parts of old Chickasaw and Muscogee Creek lands.
Split into simpler sentences: The region is not clearly defined; its exact boundaries vary by source. It falls within Choctaw Nation of Oklahoma's tribal jurisdictional area, and includes part of old Chickasaw and Muscogee Creek lands.