The Control Point
The control point (or controlled point) is a concept adopted from the NIST GCode Standard that defines a point of reference for machine tool movements. From the NIST GCode standard: The controlled point is the point whose position and rate of motion are controlled. In other words, when we put the machine in absolute mode and issue an absolute movement to (x=10, y=10, z=10), we are specifying that the control point move to this location in space relative to the machine's origin.
When the machine changes tools, the control point also changes. The net effect is that the control point is moved to the tip of the tool. For nozzles, this point can be conveniently defined as the nozzle tip. For spindles, this point could be defined as the center of the tip of the spindle. This behavior is implemented by defining tool table offsets for each tool that Jubilee will use.
In Jubilee's implementation, when no tools are defined, we define the control point as the trigger point of the ZProbe. Technically, this point could be any fixed point on the machine carriage. However, specifying the trigger point of the ZProbe is convenient as is prevents us from having to define additional offsets of the ZProbe to some other arbitrary point on the carriage that would otherwise be defined as the control point.