The following excerpt is taken from the Hasselblad historical website:

As indicated by the designation "S," the S-Planar f/5.6-120 mm is a special lens. Contrary to the usual taking lens, which is optimally corrected for large object distances, the S-Planar* is optimally corrected for close-range photography. Given this property, the
the lens is ideally suited for exceptional close-range work such as drawing reproductions and technical subjects where the standards of image quality and freedom from distortion are much higher than for pictorial photography.

The correction of the S-Planar T* is relatively invulnerable to changes in image scale. It can also be used successfully for distance shots but should be stopped further than a "normal" lens of comparable focal length. For close-range photographs, under 0.95 m adapter rings or a bellows extension should always be used, as the use of Proxars has an unfavorable effect on the S-Planar's T* correction.