.sc and .a glyph types

I've noticed that some fonts have two versions of entire glyph sets.
Adobe Garamond Premiere Pro has the small caps (extension: .sc) and an almost equal version (extension: .a, which I believe is for alternative), but in which, with the exception of very few glyphs (such as the two versions of Q or W ), all others are equal. The names change: A.a = Latin Capital Letter A and a.sc = a small cap (corporate Use), but I do not understand the function of this so wide doubling.

The thing is even more confusing to me if I look, for example, at the 'locl' lookup for the two types of Turkish <i> (dotted and dottless).

Here even two different rules are used, apparently for the same substitution, which certainly has its own precise logic, which however escapes me:
----------------
|  I.a    |  u0130.a  |

|  i       | i.dot         |

|  i.sc  |  i.dotsc     |
----------------

It is likely that I.a and i.sc have a different logical function, but the glyph is the same: the dottless small cap I. Similarly u0130.a and i.dotsc are the dotted small cap  i.

Therefore, in cases like this, what is the function of the double glyphs .a and .sc?