Wall Body Table

[Cantriped]

On the Wall Body Table (6e2 172), what is the "---" for wood walls 4mm thick or less, and stone walls 16mm thick or less intended to indicate? There isn't any rules text (which I can find) under Wall Body (6e2 173), or anywhere else I've checked, definining what these entries mean.

[Steve Long]

I've always understood it to mean "you can't have a wall of this material that's this thickness." I didn't create the table originally, so I can't answer definitively, but that seems the most logical interpretation to me.