Here are explanations of every salary differential:
C1: A bachelor’s degree
C1+PD: PD stands for “promotional differential.” The contract defines a variety of ways to get this salary increase. Generally speaking, you qualify by having a master’s or 36 credits in an approved subject matter area. In the latter case, you must also have 30 credits beyond the baccalaureate. The 36 credits in an approved subject matter area can be part of your undergraduate work, your graduate work, or a combination of both.
C2: The “first differential,” which requires 30 credits beyond the baccalaureate.
C2+ID: ID stands for “intermediate differential.” To get it, you must have 60 credits beyond the baccalaureate.
C2+PD: The first and promotional differentials.
C2+ID+PD: The first, intermediate and promotional differentials.
C6: This is the “second differential.” To get it you need a master’s degree plus 30 credits that were taken after the date of the baccalaureate and were in addition to and not part of the master’s degree.
C6+PD: The second and promotional differentials. These days, almost everyone with a master’s plus 30 credits goes directly to C6+PD.
Anyone seeking to qualify for any differential should consult DOE circulars and regulations to make sure any courses you take are acceptable for accreditation before enrolling.