Lr6 custom Lens Corrections Q:

What is the "Custom" and  "Save New Lens Profile Default" for if not to customise the profile?

And in fact that is exactly what they do. But your question and what you want to do have nothing to do with that or any lenses profile. IIRC, it was back in Vs. 4 (P.V. 2012) that the new CA Correction and Fringe Correction became separate and independent tools that were no longer part of the profiled lens corrections and functioned without regard for whether the profile was applied or not. Profiles now control only Vignetting and Distortion.

You have to manually apply the preset you saved.

There is an option to update the Camera Defaults based on the cameras serial number. But that will be for all shots from that camera and I'm not sure what you are looking for, as far as the lens correction goes.

Try saving a user preset –see image below. After setting the CA panel keep your image selected and click the plus icon on the presets tab in the left hand sidebar. After you create a new user preset it can be selected in the import dialog to be applied to all images being added to Lightroom.