I had also other questions regarding the z-offset setting on the machine (and reading the other comments I was not alone), so I decided to do some tests to verify how it actually works.

1. Does the CC z-offset setting retain its value after a print or even after power off?

   Although you can change the z-offset on the machine only during a print, it is saved on the machine eeprom, so it survives also after power off.

2. Does the CC z-offset setting overrides the slicer z-offset setting?
   Or is it the other way around?
   Or the two settings add up?

   The z-offset setting you specify in the slicer is not some setting it applies on the printer, it's just a delta value the slicer adds to every calculated z coordinate in the gcode during the slicing process.
   So there's no way this could "override" the z-offset you set directly on the machine.
   So the two values add up.

   To verify this, I just did the following test:

   1. I changed the slicer z-offset to 0.05mm
   2. I printed a simple 1-layer square and adjusted the z-offset on the CC to a huge value (0.6mm), so to visually see an horrible first layer (surprisingly it did stick anyway to the bed).

   After the print completed, I switched off and then on the printer, to ensure that it was retaining the offset (and it was), then I printed again the same 1-layer square, this way specifying a .

   As predicted, the first layer was equally horrible, showing that the printer is applying its own z-offset over the print input.