aggegation_level:
The functional size of the resource.
    aggregation_level:
        enumerated
    (
        "1",    // Level 1: Smallest level of aggregation,
            // e.g., raw media data or fragments.
        "2",    // Level 2: A collection of level 1
            // learning objects, e.g., a lesson
        "3",    // Level 3: A collection of level 2
            // learning objects, e.g., a course
        "4",    // Level 4: The largest level of granularity,
            // e.g., a set of courses that lead
            // to a certificate.
)