The precision value tells PhotoModeler during processing how accurate you think the control points are. Mathematically this number is defined as the standard deviation of the normal distribution of the error in the control points x, y and z coordinates. The smaller this number the less PhotoModeler will try to adjust the position of the control point to compensate for errors. If these control points were collected in an imprecise manner, you can have PhotoModeler adjust them along with all the other data by giving them a larger precision value.
The precision values in a TXT/CTL file (if present) will override the precision in the dialog.
If the "fixed" option is selected or a precision value is negative the corresponding control point will not be adjusted at all. In other words, it is considered to be infinitely precise. If you lock down a control point that has a poor value by using a negative precision value you will probably cause processing to fail. Control points that are fixed will appear with "fixed" in the precision field of the Imports and Coordinate Systems Pane and the Measurements Pane. Points used for inverse camera should be imported as “fixed”.