  New definitions of $Q$-conditional symmetry for systems of PDEs are
presented, which generalize the standard notation of non-classical
(conditional) symmetry. It is shown that different types of $Q$-conditional
symmetry of a system generate a hierarchy of conditional symmetry operators. A
class of two-component nonlinear RD systems is examined to demonstrate
applicability of the definitions proposed and it is shown when different
definitions of $Q$-conditional symmetry lead to the same operators.
