  Since 2003, plenty of resonant structures have been observed in the heavy
quarkonium regime. Many of them are close to the thresholds of a few pairs of
heavy hadrons. They are candidates of exotic hadrons and have attracted immense
attentions. Based on a coupled-channel nonrelativistic effective field theory,
we classify the near-threshold structures of a symmetry-related two-channel
system according to the scattering length and channel coupling strength. We
show that the evolution of the scattering amplitude line shapes can be
understood from the pole trajectories in the complex energy plane, and the pole
evolution can be traced back to the renormalization group fixed points. We
provide a dictionary of correspondence between the evolution of line shapes and
pole trajectories, which can be used to understand the experimental
observations of the near-threshold structures.
