5 Gauss is the field level the report uses as the edge of a device's useful reach. Where the report describes how far a field reaches, it measures out to the distance at which the field falls to 5 Gauss. This is a measurement reference, not a biological threshold. It was chosen so a reported reach reflects field that stands clearly above the background, and the report does not claim the field stops mattering below it. For context, the earth's natural magnetic field at the surface is about 0.3 to 0.5 Gauss, so 5 Gauss is roughly ten times the ambient background.
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.
