There's another unit later on where the noun suffix -'a' is discussed specifically.
TKD describes this suffix as:
This suffix indicates that what the noun refers to is bigger, more important, or more powerful than it would be without the suffix.
My impression is that words formed with -'a' are often lexicalised, and it's not always possible to predict the exact meaning of the suffixed noun just by knowing the meaning of the base noun.
For example, bIQ is "water" but bIQ'a' is not just any "great waters" but specifically "ocean". A Duy'a' is not just a "great emissary" but specifically an "ambassador".
Dun, on the other hand, is a regular stative verb/adjective meaning "(be) wonderful, great" and can be applied to any noun, and the meaning of the combination would be reasonably regular -- a Sub Dun would be a hero who is great or a hero who is wonderful, while a Sub'a' would, I suppose, be some class of person that is even greater or more important than a "regular" hero -- "national icon", perhaps?