ManualContour is the contouring method used when AutoContour is turned off. In this mode, contours are drawn using the ContourLevel Threshhold and ContourSpacing Factor determined with "View-Set Contour Parameters", "Planes-Set Contour Parameters" or "Strips-Set Contour Parameters".
The Threshhold determines the lowest intensity IsoIntensityContour drawn (like the contours on a topological map).
Factor determines the intensity factor separating adjacent IsoIntensityContours (geometric progression)
Therefore the first three contours will have the following intensities:
Threshold
Factor x Threshhold
Factor x Factor x Threshhold