From the RTF spec:
\abshN -- N is the height of the frame in twips. A positive number indicates the minimum height of the frame, and a negative number indicates the exact height of the frame. A value of zero indicates that the height of the frame adjusts to the contents of the frame. This is the default for frames where no height is given.
have you ever seen this sort of thing in an API where a positive value in a parameter means one thing and negative value means something very different?