0,0 refers to your starting point. For you example with the circle you could say 0,0 is your center, every other point detected is based on that center coordinate. Same goes for a checkerboard, if you have a simple checkerboard with lets say 4x3 squares (25 mm each), then your first square edge would be 0,0 and the second 25,0 then 50,0 followed by 75,0 and then 0,25, 25,25 and so on. Now if you turn that checkerboard by 45 degree your first point is still 0,0 followed by the next detected point.
So for a selfmade shape your 0,0 coordinate will be the point you detect on the most left/right top/bottom side, depending on your implementation. Every following point coordinates refers to the 0,0 as starting point. This has also nothing to do with the pixel value itself, a point on pixel [435 755] can still be coordinate 0,0 for your calibration pattern
