It is conventional to assume that the state input to the circuit is a computational basis state, usually the state consisting of all $|0\rangle$s. This rule is broken frequently in the literature on quantum computation and quantum information, but it is considered polite to inform the reader when this is the case.
What does this mean?
Source: M. Nielsen and I. Chuang. Quantum Computation and Quantum Information. Cambridge UniversityPress, 2000. (1.3.4 Quantum Circuits)