The unique one-dimensional electronic and optical properties attributed to single-walled carbon nanotubes (SWCNTs) are mainly related to the peculiar local arrangement of sp2 hybridised carbon atoms. This structural configuration gives raise to interesting features, which can be identified with various spectroscopic techniques. In the case of SWCNTs, high energy spectroscopy methods represent effective key tools to analyse the modifications of the underlying basic correlation effects in the bonding environment, the charge transfer between functionalized nanotubes, and on-wall doping. More specifically, in this article we review the shape of the C1s photoemission (PES) response related to the density of states (DOS) of the valence band (VB) in SWCNTs and its changes upon on-wall functionalization and metallicity-sorting. In the last, the progress in the identification of changes in the site selective valence-band electronic structure is clarified in detail.