Cable Liner
It is the low-friction plastic tube placed inside the Bowden sheath, on which the inner wire slides. By preventing the wire from rubbing directly against the metal sheath it lowers the actuation force and makes the movement repeatable. A special anti-friction liner is used for this purpose in the cable of the GS740 kit.
In a cable without a liner the inner wire slides directly on the metal winding. In that case friction is both high and variable, depending on the curvature of the routing and on contamination. Variable friction means that the same actuator position does not give the same throttle opening every time.
A low-friction plastic liner solves two things at once. The sliding surface becomes smooth and predictable, and since the liner is self-lubricating no separate grease application is needed. In the GS740 kit the statement that it requires no lubrication comes directly from this construction; applying grease would trap dust and swarf inside and therefore do harm instead of good.
The liner's worst enemy is crushing. When the sheath is bent and crushed somewhere, the plastic tube inside narrows and the wire binds at that point. If the cable feels stiff in the field, first check along the routing whether the sheath is crushed anywhere; most of the time the problem is not in the actuator but at a single binding point.
Context in which this term is used: Electronic speed and throttle control on a diesel engine