If the snap angle is 0, the axes of the isometric planes are 30 degrees, 90 degrees, and 150 degrees. You can work on any of three planes, each with an associated pair of axes:
Choosing one of the three isometric planes also causes Ortho and the crosshairs to be aligned along the corresponding isometric axes. For example, when Ortho is on, the points you specify align along the simulated plane you are drawing on. Therefore, you can draw the top plane, switch to the left plane to draw another side, and switch to the right plane to complete the drawing.
You can cycle quickly through the three isometric planes by pressing F5 or CTRL+E to choose the plane you want to work in.