When a user taps on a main function, it expands to show sub functions, if applicable. As with the move icon, when an item is selected, the icon and background colors are inverted.
As illustrated here, the shape function expands just like the pen does. Users can adjust the color and opacity of the shape. (When users manipulate the slider, the opacity display adjusts).
Users can create and maintain layers as they annotate the document. Layers can be shown, hidden or deleted.