The dialogue box with the core selection only is available when using interior dimensions. I noticed you are using V12, maybe it's missing in V14?
Also, I have discovered that it appears that sometimes the automatic dimensioning uses whatever is set on the default dimension dialogue box. I changed the default and found that sometimes the ext. dimensions won't accept the default. How does one change the auto dimension style, or what triggers the auto. dims to accept the default.
Walls have core and finishes defined.
Just wondering.
Tom