  We propose a range criterion which is a sufficient and necessary condition
satisfied by two pure states transformable with each other under reversible
stochastic local operations assisted with classical communication. We also
provide a systematic method for seeking all kinds of true entangled states in
the $2\times{M}\times{N}$ system, and can effectively distinguish them by means
of the range criterion. The efficiency of the criterion and the method is
exhibited by the classification of true entanglement in some types of the
tripartite systems.
