When you specify single-playfield mode you are asking that the system
treat all bitplanes as part of the definition of a single playfield image.
Each of the bitplanes defined as part of this viewport contributes data
bits that determine the color of the pixels in a single playfield.

     figure 27-12: a single-playfield display 

If you use dual-playfield mode, you can define two independent, separately
controllable playfield areas as shown on the next page.

     figure 27-13: a dual-playfield display 

In the previous figure, pfba was included in the display mode.  if pfba
had not been included, the relative priorities would have been reversed;
playfield 2 would have appeared to be behind playfield 1.