The search model is based on the Kundel-Nodine model of the diagnostic interpretation process. It predicts ROC, LROC, AFROC and FROC curves. The search model predicted ROC curves are always proper - they never cross the chance diagonal. The model is used in the curve fitting and sample-size estimation capabilities of the software.
For further details see my two papers in Physics of Medicine and Biology ; e-mail me if you need pdfs.