Dodatkowy and other 68,000 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
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.