  We describe a new type of discrete symmetry that relates heterotic-string
models. It is based on the spectral flow operator which normally acts within a
general N=(2,2) model and we use this operator to construct a map between
N=(2,0) models. The landscape of N=(2,0) models is of particular interest among
all heterotic-string models for two important reasons: Firstly, N=1 spacetime
SUSY requires (2,0) superconformal invariance and secondly, models with the
well motivated by the Standard Model SO(10) unification structure are of this
type. This idea was inspired by a new discrete symmetry in the space of
fermionic $\mathbb{Z}_2\times \mathbb{Z}_2$ heterotic-string models that
exchanges the spinors and vectors of the SO(10) GUT group, dubbed spinor-vector
duality. We will describe how to generalize this to arbitrary internal rational
Conformal Field Theories.
