The IBM-PC 8253 Timer - What do the three outputs of the timer control?

[elmerfud@wb.com (Elmer Fudd)]

Hi all!

I'm playing around with the 8253 timer and need some information on
what the three timer channels actually control on a standard IBM-PC
compatible mainboard.

I know the first channel (by default) is set in MODE 0 (interrupt on
terminal count) and is responsible for the ticks that occur 18.2
times/second, by triggering hardware interupt 0.

BUT!  What do the other channels output too? 
Are there any Guru programers that can answer this question?

Cya 
Ian S.