Eddy is the proximity sensor, that is it.
Calibration curve is the mapping between Z values and sensor’s coil frequency.
z_offset has been deprecated and renamed in the current code for Eddy sensors, btw.
z_offset describes where probe will trigger.
For normal probes it is a fixed point in space.
By so when you adjust it, you changes the mapping between trigger point to Z
For eddy it is a Frequency from the calibration curve.
So by change in the z_offset/descend_z, you change where it should trigger.
But, eddy is a proximity sensor, so there is no real hardware trigger.
So by change the “where to trigger” you do change “where to trigger”.
So, if it set to 1 mm, it will be triggered at 1 mm according to the curve and so on.
So, any adjustment to Z with babystepping should be saved inside the calibration curve, because what you meant is that previous Z position was wrong, and by so curve was wrong.
There are some (old) additional explanations: Make BTT Eddy great again
Otherwise it is a “normal probe”, so there is not a lot of difference.
There is a thermal drift, which is mentioned in the docs, which have to be accounted for.
Hope that explains something,
-Timofey