Here's an example of positive and negative displacement.

Same object, same angle, same mat, same functions. Negative displacement is on the left. Positive of the same value is on the right. Notice the differences.  I've been working on big jagged rocks and cliff faces so for my purposes, the one on the left is better suited. For a rock on a seashore or one jutting up out of a forest floor, the one on the right might be preferable.

Enough differences in appearance to warrant using both I think.

Thanks for the link!