Cross Reference Entry 

"A Cross Reference Entry enters a document under one or other of the classes other than the dominant focus or class 

For example, suppose the book is a Text Book of Physics and in this book the subject of Electronics is also presented. Then the dominant focus or class is physics but we can help the user by providing a Cross Reference Entry for the class Electronics: 

Such entries in a dictionary catalogue are known as Subject Analytical Entries. In the classified catalogue the Cross Reference is given under the class number for the other classes for which entries are to be given. (You will be knowing more about it in the course BLIS-04P Library Cataloguing Practice).