  Physical states are characterised uniquely by their pole positions and the
corresponding residues. Accordingly, in those parameters also the nature of the
states should be encoded. For bound states (poles on the real $s$-axis below
the lowest threshold on the physical sheet) there is an established criterion
formulated originally by Weinberg in the 1960s, which allows one to estimate
the amount of compact and molecular components in a given state. We demonstrate
in this paper that this criterion can be straightforwardly extended to shallow
virtual states (poles on the real $s$-axis below the lowest threshold on the
unphysical sheet) which should be classified as molecular. We argue that
predominantly non-molecular or compact states exist either as bound states or
as resonances (poles on the unphysical sheet off the real energy axis) but not
as virtual states. We also discuss the limitations of the mentioned
classification scheme.
