I am not sure if this value is part of the Dicom standard or it is just a specific value for a given manufacturer.
I am telling that because I only have seen these values for the images generated by an old (currently out of service) Agfa ADC compact CR
In the documentation you can read this:
LOG_E REL: pixel values are linearly related to the Log Exposure on the image plate; the maximum pixel value corresponds to a delta LogE of 3.2767 above the LogE for the minimum pixel value; in this case, a VOI module (sequenced item) is present, also containing a lookup table. Only 12 bit is supported.
I do not know if you should apply a rescaling formula or this is just a note related to some kind of postprocessing algorithm having been applied to the original image.
I assume you should just apply the given VOI LUT instead of trying to apply a rescale equation.