The boundaries of expression of hox genes often fall on segment boundaries or other morphologically distinct boundaries. In vertebrates each gene has a sharp anterior border of expression and, usually, an extended posterior domain. Expression domains of adjacent genes in a single cluster are partially nested . Let us take this statement as an hypothesis. By the hypothesis, hox genes figure importantly, causally, in this AP identity coding.
Individual vertebrae can have identity.
E.g. the first and second vertebra in the neck of a tetrapod are specialized as the atlas and axis, permitting functional articulation with the skull. Specialization of the two, in mammals, at least, provides for fine control of head movement as well as strength of the joint. Heterosis and homeosis. In 1894 Bateson coined two terms, as applied to animals with segmental body plans. In the 1950's Ed Lewis showed that loss of function mutation of Hox genes in flies often produced homeosis.