The fourth selection at the bottom of the page is another I/O module, allowing for 4 analog outputs.
AQ0
Burner Control Signal 3
This is an analog output used to control the opening of the gas control valve for burner #3.
AQ1
Burner Control Signal 4
This is an analog output used to control the opening of the gas control valve for burner #4.
AQ2
Burner Control Signal 5
This is an analog output used to control the opening of the gas control valve for burner #5.
AQ3
Burner Control Signal 6
This is an analog output used to control the opening of the gas control valve for burner #6.