GEM Subject is a subject taxonomy created and maintained as part of the Gateway to Educational Materials project. It is relatively broad and shallow, with 238 classifications, and so may be easier to navigate.

LC Classification (AKA LCC) is a subject taxonomy created and maintained by the Library of Congress. It is very fine-grained and very deep, with more than 430,000 classifications, and so provides for extremely accurate and specific categorization of resources.

Key Concept is a subject taxonomy that Internet Scout has developed to support the AMSER collection. The terms are the specific, fundamental bits that make up more complex knowledge in a given field, designed to help educators local materials that convey these key concepts to students in the classroom.