This is a gauge of how much of the available transformer window is expected to be full after the transformer has been wound. The Fill Factor accounts for insulation, tape, termination space as well as open spaces found in a typical bobbin wound transformer. Thus a FIT of 100% corresponds to a completely filled winding window. Actual results may vary from the estimated values and may depend on how tightly transformer windings are wound, variations in thickness of tape that is used, method of terminations etc. Referring to the figure below, observe that the windings protrude out of the bobbin edge. This corresponds to a Fill Factor (FF) of > 100%.
Illustration shows the impact of Fill Factor being > 100%.