The Selected Material icon displays and determines the material of all drawn strokes and objects. You can click this picker to use the current Material palette settings, or click and drag to the canvas to select the material at that point.
The Once Material button causes brushstrokes and objects to pick material from the canvas when first clicked, and hold that material throughout the following brushstroke.
The Continuous Material button causes brushstrokes and objects to change their material to the materials already on the canvas, continuously while dragging.
Reference Guide > Picker> Material