The FAB (old) classification of MDS, from Wikipedia

MDS is classified according to bone marrow findings:

• RA = refractive anaemia
  Myeloblasts less than 5%.
  Sideroblasts less than 15%.
  Pathology mostly confined to erythroid precursors.
• RARS = RA with ringed sideroblasts
  Myeloblasts less than 5%.
  Sideroblasts more than 15%.
• RAEB = RA with excess blasts
  Myeloblasts 5-20%.
• RAEBt = RAEB in transformation
  Myeloblasts 21-30% (more than 30% blasts = AML, by definition).
  
• CMML = chronic myelomonocytic leukaemia
  5-20% blasts + monocytes > 1000x10⁹cells/µL.

In the newer WHO classification CMML is no longer considered a MDS, but an overlap between MDS and MPD.