2.1.7 [W3C-EPUB3] Section 3.4.12. The spine Element
V0011: If the navigation document is not present in an EPUB3 publication, the NCX is used
The specification states:
› 3.4.12 The spine Element The spine element defines the default reading order of the given Rendition of the EPUB Publication content by defining an ordered list of manifest item references. ... › NCX Superseded The NCX feature defined in [OPF2] is superseded by the EPUB Navigation Document [ContentDocs301]. EPUB 3 Publications may include an NCX (as defined in OPF 2.0.1) for EPUB 2 Reading System forwards compatibility purposes, but EPUB 3 Reading Systems must ignore the NCX.
If the navigation document is not present in an EPUB3 publication, the NCX is used.