digital cable

What is the difference between 75 OHM digital cable to 110 OHM digital cable, are they sound the same? , does they have a different purposes?, what is the difference between them?.can I use 110 OHM digital cable despite the manufacturer's instructions to use 75 OHM d.c?
Good question! And what is the difference between a digital cable and an "analog" one? What does the 75 ohm and 110 ohm resistance apply to -- the cable, or the source connection? I've tried half-pair of analog that sound better than some premium digital cables.
110 ohms is the "industry standard" impedance for an XLR connection. 75 ohms is the "industry standard" for a single ended digital connection. If the device has jacks for both XLR and RCA, you can probably use either. How well it works out of either is up to how much attention the manufacturer paid to designing the unit.

As far as digital vs analogue cables go, some analogue cables will work very well as a digital cable, others won't. The only way to find out is to try them and see. Due to high levels of reactance, poor shielding, etc... some analogue cables may not allow the DAC to lock onto the signal from the transport and / or spray the RF based digital signal around due to leakage from the cable.

As a side note, i've found that many analogue cables retain specific sonic traits when used as a digital cable. Kind of weird, but i have experienced this first hand. Sean
Sean hit the nail on the head. I've used singled ended digital links that stated 75 ohm, but the dac would not lock on correctly. I only use Goldmund Lineal digital cable, which is truly 75 ohm.