Signal Cables

I recommend you research common mode voltage for an explanation on why analog signals use a twisted pair of wires. (Big hint: Interfering signals will be evenly transmitted onto each wire of a twisted pair.)

Now why digital signals use coaxial or transmission line cable requires a more in depth understanding of mathematics and signal propagation. First you must understand the Fourier transform of a digital signal implies that a 1 Mhz square wave has significant power harmonics well into the 10 Mhz range and that to have a usable square wave at the end of th cable you need all of these frequencies to propagate down the cable at the same velocity with minimal reflections.

Er, because it says so in the Client's Cabling Specification?