Range and Azimuth shift apply the enhanced spectral diversity (see references).
Basically it takes advantage of the overlap between bursts to fine tune the coregistration. Generally the backgeocoding is accurate and the ESD can provide a pixel or sub-pixel shift to improve the coregistration.
P. Prats-Iraola, R. Scheiber, L. Marotti, S. Wollstadt, and A. Reigber, “TOPS interferometry with TerraSAR-X,” IEEE Trans. Geosci. Remote Sens., vol. 50, no. 8, pp. 3179–3188, 2012
R. Scheiber and A. Moreira, “Coregistration of interferometric SAR images using spectral diversity,” IEEE Trans. Geosci. Remote Sensing, vol. 38, no. 5, pp. 2179–2191, July 2000