The present work deals with the system-bath separation of the vibrational modes involved in a nonadiabatic system. System modes are the strongly interacting modes which dominate the overall dynamics and, hence, need a near-exact treatment. Bath modes have relatively weaker couplings and can be approximately treated. Thus, the exponential bottleneck involved in computations is controlled by the size of the system-subspace. The aim of this work is to put forward a set of criteria that provides clear guidelines for choosing the system degrees of freedom. The extent of wave packet dephasing caused by repeated crossings across the surface of curve-crossing is the basis for distinction between system and bath modes. The mechanisms of the wave packet dephasing and the criteria are discussed in detail. Numerically converged results presented for the 24-mode pyrazine and 3-mode spin-boson model validate the efficiency of these criteria.