In the last decades, the number of exotic candidates which are beyond the conventional quark model has grown dramatically.
At the same time, numerous theoretical interpretations, such as tetraquark, hybrid,
hadroquarkonium and hadronic molecule, have been proposed to understand
their nature. In principle, all the configurations with the same quantum numbers
can mix with each other.
Thus at present we aim at identifying the prominent component.
As an extended object, a hadronic
molecule has some distinguishable features from other more compact configurations.
In the end, the two $Z_b$ states are used to illustrate some features of the molecular scenario.