When a book mentions something from a different part of the same book, that's a cross-reference. In a textbook, a cross-reference might look like this: "See page 123."

The Boiler Control 256 is designed to operate a single boiler using Outdoor Temperature Reset. It can be used in applications ranging from a single zone of baseboard to multiple zones of fan coils and baseboard. This control regulates a single heating water temperature through Outdoor Temperature Reset.

Cross-reference, "a reference in a book to another part of it," was coined in the 19th century. It's a way to add more information without going into wordy detail, similar to using a footnote or endnote in academic writing. If you look up "Amelia Earhart" in an index at the back of a book, you might see cross-references such as "See also: Earhart, Amelia" and "See also: aviators."