I consider the upper and lower bounds of the overlap band to be the frequencies where the stop band driver is attenuated 6dB. It's a fuzzy area though, because the real issue is whether there is enough energy between the two drivers to cause cancellation at the null angles. Deep into the stop band, a driver can't make any sound, so nulls don't form. Only in the overlap band where both adjacent sound sources are online will nulls form. A symmetrical crossover is one that has matching low-pass and high-pass slopes. An asymmetrical one has staggered slopes or frequencies or both.