"The pixels on the edge are supposed to be halfway between white and black to indicate being half-covered by the circle, but if you pile them up on top of each other anything with a little black adds up with its duplicates to complete black."
The best docs I could find in a short amount of time, are those from anti-grain[1] where the author describes it as a "second alpha channel.
[1] http://agg.sourceforge.net/antigrain.com/doc/basic_renderers...