Criteria for dictionary inclusion - what is it?

Good (Morning|Afternoon|Evening)!

I was just presented with a word that appears not in the aspell
dictionaries.  That got me wondering - what is the criteria used to
determine words to include in the aspell dictionary? I skimmed the
manual, searched the list archives, and have done a general Internet
search - yet I could not find the answer.

Maybe somebody here knows and is willing to share?

Thanks!

--Don