Idea of Additional Icon
The icon is very simply composed of an empty circle with a large, upper case letter A in the center, about as wide as the circle's radius. The icon stands for the presence of additional elements outside of the ones currently being represented on the screen.
This Additional icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 6,200 icons, all in the same style and quality.