  We consider the string theory origin of R-charge conservation laws in
heterotic orbifold compactifications, deriving the corresponding string
coupling selection rule for factorizable and non-factorizable orbifolds, with
prime ordered and non-prime ordered point groups. R-charge conservation arises
due to symmetries among the worldsheet instantons that can mediate the
couplings. Among our results is a previously missed non-trivial contribution to
the conserved R-charges from the Gamma-phases in non-prime orbifolds, which
weakens the R-charge selection rule. Symmetries among the worldsheet instantons
can also lead to additional selection rules for some couplings. We make a
similar analysis for Rule 4 or the 'torus lattice selection rule'. Moreover, we
identify a new string selection rule, that we call Rule 6 or the 'coset vector
selection rule'.
