Hi Kai, clocking is what you think it is. A "Dial Test Indicator" (clock/dti) is an indicator that shows physical movent on a dial. We spoke about this on the weekend, but i'm sure you've forgotten most of what was said.
The most common use on the mill would be, setting the vice up so that it's square to the x axis. You do this by clamping a parallel (or piece of flat metal) in the vice jaw and setting the clock to show the amount of movement along the face of the parallel when you move the x axis. You then adjust the position of the vice so that there is zero movement on the clock from end to end of the parallel.