Stack Overview question

Dear All,
I am trying to apply some interferometry to create a DEM and I am following the “Interferometry Tutorial with Radarsat-2”.
Before co-registering the two images, I have launched the Stack Overview tool, in order to take a look at the Baseline and the other parameters listed.
There is one thing that I don’t understand: from what I get from the Online Help, the tool calculates a “modeled coherence” in order to choose which master is best. Then, the values of the Modeled Coherence that I can read for the two images, what do they actually mean? For example, image1 has been chosen as master and has a Modeled Coherence of 1.00, while image2 is slave and has a MC of 0.59.
What will be the coherence of the stack? 1.00 because image1 is Master, or 0.59?

Another question: what is Height Ambg?

Thank you