One of the factors limiting total cable length is the capacitive load. Systems with long lengths benefit from using low capacitance cable. The speed at which an electrical signal travels in the cable. Plenum cable Plenum rated cable is fire resistant and less toxic when burning than non-plenum rated cable. Check building and fire codes for requirements.
Plenum cable is generally more expensive due to the sheathing material used.
The RS-422 specification recommends 24AWG twisted pair cable with a shunt capacitance of 16 pF per foot and 100 ohm characteristic impedance. It can be difficult to quantify whether shielding is required in a particular system or not, until problems arise. We recommend erring on the safe side and using shielded cable. Shielded cable is only slightly more expensive than unshielded. Another choice is the same cable commonly used in the twisted pair Ethernet cabling.