[1][2] A type 1 sequence boundary is defined to be a sequence boundary "characterized by subaerial exposure and concurrent subaerial erosion associated with stream rejuvenation, a basinward shift of facies, a downward shift in coastal onlap, and onlap of overlying strata".
[3] Similarly, a type 2 sequence boundary is "marked by subaerial exposure and a downward shift in coastal onlap landward of the depositional-shoreline break; however, it lacks both subaerial erosion associated with stream rejuvenation and a basinward shift in facies"[3] So the main distinction between type 1 and type 2 sequence boundaries is the amount of subaerial exposure.
[4] Since they were hard to recognize, they were redefined in 1990 by Van Wagoner et al..
However even with this new definition, type 2 sequence boundaries were hard to recognize in the field due to their lack of subaerial exposure.
Therefore, the abandonment of this nomenclature was recommended by Posamentier and Allen in 1999.