The ROIDistanceUnits property of a DicomLabel varies according to the attribute used to obtain the scaling information, for which there are 5 possibilities. Normally only one of these should be present, but should there be >1 then the first in this list is used. In all cases of course, the values returned by other ROI functions (ROILength etc.) are in matching units.
This is found in some ultrasound images, giving different calibration for different regions of the image. A measurement which fits entirely with one such region is returned either as "mm" or, if the units are not appropriate, or if crosses regions, then the returned value will be "pixels"
This is commonly used in CT, MR and US, being derived from DICOM attribute 0028,0030, giving a measurement in "mm"
This applies to all forms of projection imaging (plain films, angiography etc.) which suffer from the effects of geometric_magnification. In this case, the true size of an object cannot be known, only the size of its shadow on the imaging system (plate/sensor), and the result is then "mm at imager". The associated DICOM attribute is 0018,1164.
This is only used in radiotherapy, where (unlike diagnostic films) there is a defined "Image Plane" within the irradiated volume. In this case the spacing comes from DICOM attribute 3002,0011 and the units are returned as "mm at Imaging Plane"
If none of the above attributes are present in the image, then measurements may only be made in "pixels"