In this paper, we introduce a new test generation complexity notation called τ^k notation, which consists of τ^k-equivalent and τ^k-bounded, in order to clarify the classification of sequential circuits based on combinational test generation complexity. We reconsider the test generation complexity for the existing classes of acyclic sequential circuits. Several new classes of sequential circuits that cover some cyclic sequential circuits have been identified as being τ-equivalent and τ-bounded.