Files with the NCX extension are associated with electronic publications stored in EPUB files. The NCX is part of the EPUB archive, which contains basic information about the e-book such as title and author, as well as a kind of electronic table of contents that allows you to easily reach defined places in the digital book. From the technical point of view, NCX is a text file containing code created in accordance with the OPF standard. This format is based on XML.

NCX files can be opened and modified by the user in any text editor.