Your PCB is designed by a CAD program. The CAD program uses its own coordinates; Most often, the origin is at the board low left corner. This is also the recommended location. When placing parts, the software needs to know the parts target location, in machine coordinates. To find out these machine coordinates, visual measurement process is used.
When building the job data, components with same value and footprint are collected together on one row. One of these rows should have data for alignment marks, usually called “fiducials”. Please see PCB requirements page for a description of industry standard ficucials. Using these makes the process more reliable, but is not mandatory, other PCB features, such as pads or vias, can be used as reference marks also. If you name your fiducials as FI1, FI2, … or FID1, FID2, … the program automatically recognizes these (case insensitive); otherwise you need to indicate the row by setting the method as “Fiducials”.
The idea is to visually measure the the location (machine coordinates) of the alignment marks. From these, the program will construct a mathematical transform between the CAD data and machine coordinates. Four reference marks, spread out on the board, will results to most accurate results. Two are needed at minimum. The accuracy of the transform will suffer the farther away you get from the area defined by the reference marks.
The machine will go trough the alignment marks one by one. First it goes to the nominal location, which is just the CAD data coordinates plus the PCB position. Assuming all goes as planned, it takes an initial measurement. From this it gets on top of the mark or a more accurate data, re-trying a few times if needed. After measurement, the program pauses briefly on top of each one. I recommend you watch this process, in case the process picks up a wrong feature on your PCB.
After the measurement, the software builds the transform and applies it to each component. Results go to CAD data table, machine coordinates columns.
The process is fairly robust, but if it fails, you’ll get a message. A known issue is that if your board has relatively large offset (CAD data origin not close to the board), the algorithm has troubles to build a stable transform. In these cases. you can either modify the location of the origin in your CAD data, or process the CAD data in a spreadsheet program, such as Excel. (The CAD data is a comma separated text file, that all spreadsheet programs can read and write.)