Real Time and Latency

Linuxcnc's waypoints are defined in device units (mm or inch)
the STEP_SCALE defines the number of steps per device unit
All of the planning and waypoints are in device units.

The base thread is used to generate the steps. Think of the thread as being an interrupt timer routine that is called (cycled) at precise intervals (frequency). The simplest step generator is a square wave where a pin is turned on for a number of cycles and then turned off for the same number of cycles. (In practice Linuxcnc's step generator is more complex than this as it includes step length and step spacing parameters). But in our simple software step generator, the maximum step rate is the frequency/2. 

A Mesa card offloads this step generation to hardware and all Linuxcnc has to do is once every servo thread cycle is to update the frequency the card is generating. In this case the base thread is not required.