You can create a crossreferenced item using the BibTeX `Crossref' field in various ways. To manually add a crossreference to an existing entry, add the `Crossref' field to the entry in the editor, then enter the cite key of the parent item. As an alternative, you can hold down the command and option keys and drag the parent item from the main publication list into the editor window of the child item.
To create a new crossreferenced item based on an existing item, you can also select the parent item in the publication list and use "New Publication With Crossref" (keyboard shortcut ⌥⌘N) from the "Publication" menu or contextual menu. Alternatively, you can press the "New Publication" toolbar button while holding the Option key down.
Crossreferenced fields which are inherited from the parent publication are shown as gray text in the editor, or light blue if they are macro values. Inherited fields can also be recognized by the gray arrow button in the editor field, which will open the parent item for editing.