Circle of Confusion based on the camera sensor's photo-sites size:
Copy to the dof table, line #

Safety coefficient:

"Digital" approach to calculations - it takes a least side size of the 2 photo-sites of a sensor. Two, because on cameras with Bayer filter 1 pixel is created by a square of 2x2 photo-sites. Safety coefficient is just an additional multiplier for the results.

Classic calculations of the circle of confusion:
Copy to the dof table, line #

Sensor diagonal divided by:

"Classic approach" - this way ignores the size of photo-sites and number of megapixels completely. Instead it's using only the sensor's diagonal. Older standard says it should be divided by 1000, newer says it should be divided by 1500. In reality, if you work with a digital image, if you crop and magnify, this way of calculations becomes absolutely meaningless, but many people still like it. You can read more about this method here: http://en.wikipedia.org/wiki/Zeiss_formula, And much more here (chapter: "The diameter of the circle of confusion"): Depth of Field and Bokeh