Looking at the chart for spectral type peculiarities (here), I noticed that the letter a was missing from the chart. This seems to be an error, as the star Vega has spectral type A0Va, with the a definitely being a peculiarity. What does the a in spectral types mean, and is this actually a spectral peculiarity?
If present, an a (or b or ab) do not refer to spectral peculiarities but are part of the luminosity class definition explained further up on the page.
Occasionally, letters a and b are applied to luminosity classes other than supergiants; for example, a giant star slightly less luminous than typical may be given a luminosity class of IIIb, while a luminosity class IIIa indicates a star slightly brighter than a typical giant.