Whats a Combination Valve??

[95K1500]

Im going to change my brake fluid this week on my 1995 Chevy 1500 4x4. It has 4 wheel abs brakes. I was reading the chilton manual on how to bleed the brakes. Step 5 says that the combination valve must be held open during the bleeding process and to use a clip, tape, or assistant to hold the metering pin in. Im not sure what this valve is or where this pin is that I need to hold in. There is a photo for the older rear only abs system but it doesnt look like mine.
Do you need to do this step? All im doing is swaping old fluid for the new.
I have bleed brakes before just never on a car with abs so this is new to me.

Thanks for any suggestions.