What does the "a" at the end of a spectral type mean?
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.[27]