I think your hesitation is mainly a matter of vocabulary, the semantics behind the “short” word used in the Insert
menu. Think of bookmark as a definition and cross-reference as a usage for this definition.
Bookmarks are entered into a dictionary of locations in your document. Bookmarks are defined either explicitly through Insert
>Bookmark
or implicitly by several LO Writer features like (outline) chapter numbering, aka. headings, or numbered paragraphs.
When you insert a reference in your document, you can chose in the dialog which bookmark attribute you want displayed. Depending on the type of bookmark (custom, heading, …), the number of attributes varies but the page of the bookmark location is always available, maybe also the chapter. Heading bookmarks are the richest.
The main distinction is: you define once a bookmark and you refer to it many times with a cross-reference.
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EDIT 2018-01-31 following @catbill’s question in comment
Bookmark: a run of characters in the present document receiving a custom name chosen by user; the properties of this location (page number, chapter number, content, …) are recorded for later use.
Note: if the run has length zero, i.e. this is just a spot location, content will be empty of course but other properties are captured as expected. From my experiments, the run of characters is limited to the next paragraph mark or page break, whichever commes first, in case you try to bookmark a run straddling several paragraphs, so that the recorded properties are well defined.
Field: a placeholder replaced by some data computed by LibO; the data is selected in a dialog with many choices. Some shortcuts are offered in menus, like Insert
>Fields
>Page Number
.
Cross-reference: a specific variant of field where Insert
>Cross-reference
opens the “insert field” dialog in the Cross-references tab where you can select bookmark or heading (primary usage) or any numbered paragraph.
Hyperlink: in principle a reference external to the document (as http:, ftp:, mailto:, file:, etc.) but may also reference “objects” as listed in the navigator panel.
Both cross-references and hyperlinks, unless disabled, warp you to the destination when clicked. Additionally, hyperlinks can take the appearance of a button in complement to the usual “sensitive” text.
The relationship between them is quite simple. Bookmarks (and navigator “objects” in case of hyperlinks) are destinations. Cross-references and hyperlinks are sources or departures for jumps.
When Writer is used to create a Web page, you have no cross-reference because <a>
HTML elements are used both for internal jumps (no URL, only a #
fragment designating an id=
) and external jumps (“full” URL with or without #
fragment). You play with fields, bookmarks and hyperlinks.