The report gives the distance at which the field falls to 50 percent and to 10 percent of its peak, and defines the Effective Field Diameter as twice the 10 percent distance.
Where it appears in the report: Field Coverage, secondary field metrics
The evidence
The report reads these distances by interpolating between the actual measured points, not from a model. Effective Field Diameter is our definition of the useful width of the field, chosen so it measures consistently across accessories of different shapes.
The field is strongest right at the coil and weakens with distance in every direction. On the on-axis dipole model the report uses, it reaches half its peak strength at about three quarters of a coil radius from center and one tenth at about two radii. A larger coil holds the field up farther out.
