D50 means black body radiation of an object at 5000 degrees K, and includes lots of red, which looks “natural”. D65 is bluer, more like “cloudy days”, but human eyes are more sensitive to blue. While monitors seem to be in the D50-D100 range, light boxes are about D110 (11000K).
The ICC PCS always uses a white point of D50.
In an ICC Input Profile, the chromaticAdaptationTag encodes a conversion of an XYZ color from the actual illumination source to the PCS illuminant (D50), and may be useful if the actual illumination source is not D50. The actual illumination source may also be defined in the mediaWhitePointTag. However, with a perceptual rendering intent, neither of these tags are required to be used by the color management system, nor do they have any specified rendering behavior (as opposed to their use with absolute and relative colorimetric rendering intents).
It is beyond the scope of DICOM to define a required or suggested white point for rendering, since an appropriate choice depends on a knowledge of the display device or media characteristics and the viewing environment.