A decoration is a symbol that can be used to decorate a group object. Its name must start with the string decoration/, and it should contain either a path object or a group of path objects.
Ipe resizes these path objects so that they fit nicely around the bounding box of the group object being decorated. For this to work correctly, the decoration object must be drawn such that it decorates the rectangle with corners at (100, 100) and (300, 200).
To make a decoration symbol, follow these steps:
<path stroke="black" fill="lightblue"> 100 100 m 300 100 l 300 200 l 100 200 l h </path>
For inspiration, have a look at the decoration symbols in the stylesheet decorations.isy that comes with Ipe.