I would like to run the terrain correction only for the slave product of an input stack of master-slave products, so that in the output I would like to get only the corrected slave product. What should I select in the Terrain Correction panel? or which options should I enable/disable?
In the processing Parameters tab of the Terrain Correction module, you can select the bands you want to export
I guess you mean in the list of the Source Bands panel. From this list do we select only the bands to export or also the ones we want to involve in the processing?
I’m not sure what you mean by that.
If you only want export the Intensity_VV_slv1 for example, just select Intensity_VV_slv1 in the Source Bands pane 
yes, sometimes it is not quite easy to explain about these details. For example in my case, besides the master and slave intensities I also have the I and Q components for both of them. They do not give any output, but can they be selected? and in which cases? Thanks for your help
Ok I understand.
SNAP mainly works with I and Q. They are hard stored in your computer. Intensity is a virtual band, meaning that it is a band created on the fly with a very simple operation using I and Q.
In the Source Bands pane, you can of course select I and Q. But SNAP wants you to select both (one is useless without the other after all). You can also select Intensity alone if you want. Then the Intensity band will be hard encoded on your computer (not a virtual band anymore).
This is clearer indeed now. You really provided me useful details.One more doubt is still remaining: I have tried to get the Terrain Correction for the slave product in two different modes, one selecting only the Slave Intensity in the Source Bands Panel, and the other selecting also its I and Q components. The result seems to be the same, but the computing time was quite different and , in particular, higher for the 2nd case. How can that be explained?
I’ve just tried it now and yes you’re right. I don’t know why it it is the case.
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Well you really gave me good info anyway, now let’s see if I can have some more help from @mengdahl @lveci @ABraun
you could calibrate before the coregistration to retrieve Sigma0 (and to get rid of the i+q bands) and then try again to select only the slave image.
Dear @ABraun this is a good option,but my point is:does the terrain correction processing change if the I and Q components are selected in the Source Bands panel or not? As I wrote in a previous reply, I tried that processing including them in the Source Bands panel and then without including them. The results in terms of Intensity for the slave product were the same but the processing time was really different between the two cases. Do you know @mengdahl how this had been developed?
There is no good reason to terrain correct I and Q and the end result will not be meaningful. Produce your intensity and coherence images in radar geometry and terrain correct afterwards.
So if I want to terrain correct only the Intensity of the Slave product I should select only the Slave Intensity in the Terrain Correction Source Bands Panel, this is what I can desume from your suggestion
just to specify, in my processing the Terrain Correction is at the end of the pipeline. What is not clear, also because it is missing in the help section, is what does “Source Bands” mean.
Source Band: All bands (real or virtual) of the source product. User can select one or more bands for calibration. If no bands are selected, then by default all bands are used.
I have not it really clear from the Help section of SNAP