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I don't know if I am answering ok and to the point but this is what I know on that. (I have been away from CATIA for 7 months and I am not able to look into it to comment exactly...)
There is a function to divide a part surface into cavity and core (and other, such as sliders), but you need to be in the CCV (Core and Cavity Design) workbench. Problem for this is that this function doesn't exactly separate the part surface as you want to. You need to do some manual process on the surface elements of the part. (I used V5R18.)
Another way I have experienced is creating a parting line to divide the part surface into core and cavity and sliders. The point about this process is that parting line should be joined with all the funtional holes to split the part surface with. (The splitting part surface elements should be all joined, too.) This way, there is no need of having the CCV workbench module but just the GSD and the part design workbench. But the problem here is you need to create a parting line every time in order to make it automatic to get core and cavity.
I am not sure if it works with macro I am not deeply experienced with, but that's what I know of.