Terrain Model modification: What is a Drape Boundary? How does this differ from a regular Boundary?

I'm a little confused at the difference between these two terms.  Usually features with the word "Drape" only need to be defined horizontally - which doesn't seem to be the case with a Drape Boundary.  I can not seem to add either a 2D or 3D element to a Terrain Model as a Drape Boundary.  Is this feature broken?