You may want to make a copy of an item descriptor without doing any dynamic typing of that item descriptor. If so, you can use the copy-from attribute. This creates a copy of the specified item descriptor, to which you can add additional properties. This is a way to share property definitions between different item descriptors, or perhaps to add additional properties to an existing item descriptor definition. However, since these item descriptors will share the same tables (and thus the same data), it may not be wise to use both the old and new item descriptors. Instead, it may be preferable to use the super-type attribute and sub-type-property attribute if you want to use both.
