The description is in the Help now (I was on holiday when the spec came through) and will be published with the next release. The text reads:
Shape Field
Click on the drop-down arrow and select from these options:
- 'Rectangle' - if you have previously switched from the default rectangular border with sharp corners, return to that default
- 'Rounded Rectangle' - set the shape to a rectangle with rounded corners
- 'Ellipse' - set the shape to a circle or oval to accommodate the enclosed elements
- 'User Defined' - enable setting drag-points on the border to create a custom shape (see the Customize System Boundary section); this option disables the 'Horizontal Swim Lanes' and 'Vertical Swim Lanes' fields
Customize System BoundaryWhen you have selected the 'User Defined' option in the 'Shape' field, you can add way-points to the sides of the System Boundary, to drag in a vertical or horizontal direction. This helps you to create shapes that enclose more-scattered elements than could be enclosed in a rectangle or ellipse.
To set a way-point on an edge:
o Ctrl+click on the edge
To clear a way-point:
o Ctrl+click on it
To move a way point:
o Move the cursor onto it until it changes to an 'up-down' arrow or 'left-right' arrow, and move the way-point
in the appropriate direction; the boundary is orthogonal, maintaining horizontal and vertical lines during
movement