“General and specific determiners
Determiners are words which come at the beginning of the noun phrase.
They tell us whether the noun phrase is specific or general.
Determiners are either specific or general”
http://learnenglish.britishcouncil.org/en/english-grammar/determiners-and-quantifiers
“The general determiners are: a; an; any; another; other; what”
(The specific determiners are: the, my, your, his, her, its; our, their, whose,this, that, these, those, which.)
The indefinite article belongs to the general determiners. It is a type of the general determiners. It has a sort of “determinative” element. You can safely say that the “indefinite” article is “determinative”.
(Edited.)