It seems local defringe @100 is much subtler correction than global @20.
How do the global / local amounts work???
To be more specific:
Purple/Green defringing can be applied globally (amount 0 to 20) or adjusted locally (-100 to +100).
But a local of +100 has considerably less "oomph" (strength of correction) than the global at 20.
I don't know the answer to your question, but my experience tells me it doesn't really matter because, to me, only negative local is really useful. Apply whatever global is needed to eliminate the problem and use local -100 with automask to "mask" the areas where it removes color that was really there in the original scene but just happened to be within the edge and color range selected in the global controls.
It seems to me that different combinations of removing global amount using negative locals and adding to global amount using positive locals will be optimal for different photos. Presumably Adobe agrees or they wouldn't have the local slider going both directions. I just don't have a good feel for the relationship between the local and global amounts yet...
I think I read Eric Chan discussing this as follows:
if you need more "oomph" than the local +100 gives you, then you need to go global to +20 if need be, while "protecting" areas of the image that you don't want to have CA applied to, by applying local adjustments at -100 (this protects these parts of the image from the global slider setting)
Here is the section which answers my question:
The global Defringe control is sufficient in many cases, but sometimes local refinement is required. One reason is the need to “protect” certain scene colors (prevent them from being defringed). Another reason is to help suppress some minor residual fringing in aparticular area. For these reasons, Defringe is also available as a local adjustment.
-1 to -100 masks some to all of global purple+green defringing.
+1 to +100 defringes "indescriminately" (is completely independent of global purple and green defringe settings).
My new experience:
I've found both masking the global with a negative local and defringing with a positive local to be very useful.