"Save in dB" in TerrainCorrection dialogue

My typical processing steps are (ML>)Calib>Spk>TC>LindB and I see that this is also pretty much the order of steps you indicate in your tutorial. http://sentinel1.s3.amazonaws.com/docs/S1TBX%20SAR%20Basics%20Tutorial.pdf

The TerrainCorrection dialogue has the option of chosing “apply radiometric normalization” > “save as Sigma0”

What is the difference between a separate calibration and then TerrainCorrection, or only applying radiometric normalization in this TC dialogie? Without calibration before?

Any reason to chose one or the other?

The normalization in TC is sort of similar to terrain flattening where it considers the local incidence angle. Calibration in the case of S1 only applies the LUT. If your processing consists of classifying first in SAR geometry then you should probably calibrate first.