Cross-references are indications included at an index entry which direct the index user to another entry.
These are of two types mandatory ("see" references) and optional ("see also")
SEE cross-references are those which direct the user from the current index heading, which does not point to any pages, to a different index heading, which does have pages. For example:
In this example, the term "planes" is ambiguous and so the index user is directed to go to a more precise heading.
SEE ALSO cross-references are those which direct the user from the current index heading, which does point to pages, to a different index heading which points to more pages which may, or may not, be of interest. For example:
In this example, the user may be interested only in the topic of computer viruses, but they might well be interested in the other topics too.