Exploer CCD vacuum line

Just bought a 1991 2 door Explorer with auto tranny, 4 WD, and many power options (windows, mirrors, criuse control) I have never owned such a "modern" car (coming form air cooled VW's)

At any rate, one of the vacuum lines that comes off a fitting on driverside of intake (one of about seven hoses) is broken, the manual (Haynes) calls it a CCD vacuum line

What does the "CCD" stand for?

i will fix the line, but I sure wish to know what it is for.

thanks