Often, but not always, a printed document should be designed so that it can be read sequentially. People tire of flipping back and forth to find information that should be presented to them as they need it.
However, in any document, some information will be too detailed for the current context, or incidental to it; use cross references to provide access to such information. Also, an online help system or a reference manual is not like a novel; few read such documents in sequence from beginning to end. Instead, people look up what they need. For this reason, such creations should contain many cross references to help readers find other information that they may not have read.
In a printed manual, a cross reference results in a page reference, unless it is to another manual altogether, in which case the cross reference names that manual.
In Info, a cross reference results in an entry that you can follow using the Info ‘f’ command. (See Following cross-references in Info.)
In HTML, a cross reference results in an hyperlink.
The various cross reference commands use nodes (or anchors, see @anchor) to define cross reference locations. This is evident in Info and HTML, in which a cross reference takes you to the specified location.
TeX also needs nodes to define cross reference locations, but the
action is less obvious. When TeX generates a DVI file, it records
each node’s page number and uses the page numbers in making
references. Thus, even if you are writing a manual that will only be
printed, and not used online, you must nonetheless write
lines in order to name the places to which you make cross references.