If an element is used in more than one place, it will be exported with an extra ‘id’ attribute, e.g. <object type="Button" id="id27">. A later reference to that same object will have no content other than an href attribute: <object href="#id27"/>.

Imports can use the same format: the value of the id attribute is unimportant, but must be unique within the file.

Note also that <slot> tags may also have an id attribute, but this is unrelated: slots can never be reused with href.