What is an NCX file?
The NCX file abbreviated as a Navigation Control file for XML, usually named toc.ncx. This file consists of the hierarchical table of contents for an EPUB file. The specification for NCX was developed for Digital Talking Book (DTB) and this file format is maintained by the DAISY Consortium and is not a part of the EPUB specification. The NCX file includes a mime-type of application/x-dtbncx+xml into it.
NCX Specification
The NCX file contains various kinds of XML tags. The meta tags like meta name="dtb:uid" is used to mention the ID. The meta name="dtb:depth" element is set equal to the depth of the navMap tag element. The navPoint elements can be nested to create a hierarchical table of contents. The content of navLabel is the text that appears in the table of contents generated by reading systems that use the .ncx. The content of navPoint element points to a content document listed in the manifest and can also include an element identifier
A description of certain exceptions to the NCX specification as used in EPUB. Here you can view the example of an NCX file:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE ncx PUBLIC "-//NISO//DTD ncx 2005-1//EN"
"http://www.daisy.org/z3986/2005/ncx-2005-1.dtd">
<ncx version="2005-1" xml:lang="en" xmlns="http://www.daisy.org/z3986/2005/ncx/">
<head>
<!-- The following four metadata items are required for all NCX documents,
including those that conform to the relaxed constraints of OPS 2.0 -->
<meta name="dtb:uid" content="123456789X"/> <!-- same as in .opf -->
<meta name="dtb:depth" content="1"/> <!-- 1 or higher -->
<meta name="dtb:totalPageCount" content="0"/> <!-- must be 0 -->
<meta name="dtb:maxPageNumber" content="0"/> <!-- must be 0 -->
</head>
<docTitle>
<text>Pride and Prejudice</text>
</docTitle>
<docAuthor>
<text>Austen, Jane</text>
</docAuthor>
<navMap>
<navPoint class="chapter" id="chapter1" playOrder="1">
<navLabel><text>Chapter 1</text></navLabel>
<content src="chapter1.xhtml"/>
</navPoint>
</navMap>
</ncx>