As far as I am aware, lock is the same as power. It controls how much the differential locks when the throttle is applied - for instance, when lock is at 100%, the car will get a better drive out of corners but is more likely to spin because both wheels are turning at the same rate; whereas at 0% lock, the car is less likely to spin because the outside wheel is getting more of the power, but at the same time the inside wheel may spin unnecessarily and the car will not get as much of a "push" out of the corner.
This philosophy mostly works for rear-wheel drive cars... in front-wheel drive cars it is better to have more lock because then the car will turn in better under power.