The Custom Brush tab

As you probably guessed, this option allows you to export the custom brush you are currently using. This is interesting if you want to save time (it is faster to cut a custom brush and export it than cropping an image and then export it).

You want to explain to someone how to use a panel? Click and hold the said panel (as if you wanted to move it) and press [G] on your keyboard. If go on the drawing space: surprise! Your panel is now a custom brush!