The state does not interact with any of the states of the continuum (it cannot emit and cannot be excited by any wave that came from the infinity);
Energy is real and Q factor is infinite, if there is no absorption in the system.
BICs are observed in electronic, photonic, acoustic systems. Bound states in the forbidden zone, where there are no finite solutions at infinity, are widely known (atoms, quantum dots, defects in semiconductors). For solutions in a continuum that are associated with this continuum, resonant states[1] are known, which decay (lose energy) over time. They can be excited, for example, by an incident wave with the same energy. The bound states in the continuum have real energy eigenvalues and therefore do not interact with the states of the continuous spectrum and cannot decay.[2]
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^ abMaheswari, A. Uma; Prema, P.; Shastry, C. S. (2010). "Resonant states and transmission coefficient oscillations for potential wells and barriers". American Journal of Physics. 78 (4): 412–417. Bibcode:2010AmJPh..78..412M. doi:10.1119/1.3276053.