A mathematical model that predicts how a coil's field weakens with distance below its surface. It lets the report quote penetration depths at standard reference levels (50 percent, 10 percent, and 5 G) even when the field couldn't be measured at every depth. The model treats the coil as a small magnetic dipole at the center of its winding loop. Close to the coil the field falls off roughly with the cube of distance. Farther away the falloff becomes more gradual. On loop coils the fit follows the central axis, and that's what the reported penetration depth refers to. The deepest 5 G reach sits out at the winding ring, not at the center, and it's shown separately on the 3D Field Volume.
Straight out from the coil, the field falls off with depth along a known curve, the on-axis dipole-loop model. The report fits a single measured point to that curve to recover the coil effective radius, then uses the fitted curve to estimate how the field carries into the body. Real coils fall off faster than a single ideal loop, which the fit accounts for.
