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I discuss the existence of discrete properties in the landscape of freefermionic heterotic-string vacua that were discovered via their classificationby SO(10) GUT models and its subgroups such as the Pati-Salam, Flipped SU(5)and SU(4) x SU(2) x U(1) models. The classification is carried out by fixing aset of basis vectors and varying the GGSO projection coefficients entering theone-loop partition function. The analysis of the models is facilitated byderiving algebraic expressions for the GSO projections that enable acomputerised analysis of the entire string spectrum and the scanning of largespaces of vacua. The analysis reveals discrete symmetries like thespinor-vector duality observed at the SO(10) level and the existence ofexophobic Pati-Salam vacua. Contrary to the Pati-Salam case the classificationshows that there are no exophobic Flipped SU(5) vacua with an odd number ofgenerations. It is observed that the SU(4) x SU(2) x U(1) models aresubstantially more constrained.

Exotica and discreteness in the classification of string spectra