In vertebrates each gene has a sharp anterior border of expression and a fuzzier posterior boundary. Paralogues are expressed in roughly the same AP region of the body, but not exactly the same region. Expression domains of adjacent genes 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 these 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.