you simply add or subtract some value in the band maths. If an area with no change has 15 mms in the relative displacement, you can make all pixels 15 units smaller and have the absolute displacement.
The definition is based on your knowledge on the study area. Ideally a point far from the displacement patten (not too close to the image border) with high coherence. I don’t think it’s a good idea to have it defined automatically because you lose control over the final result.