Agree w/trn -- (passive) bi-amping is about power (to the loudspeakers).
According to the OP’s current profile, the OP has "... two NAD M23’s horizontally B-Amped ...". Given that the NAD M23 has a single power supply, horizontal bi-amping means that the LF M23 drives both the left and right channel woofers from a single power source. The HF M23 is of little concern because the LF musical signals draw vastly more power than do the HF musical signals. i.e. With identical amplifiers -- e.g. 2 x M23 -- vertical bi-amping should be preferred because the LF musical signal of each channel would have a separate power source. (Again, the HF power draw is small relative to the LF power draw.) If the two amplifiers are of dual-mono construction -- e.g. two Simaudio Moon 861s -- then horizontal bi-amping is fine.
If bi-amping with non-identical amplifiers, then horizontal bi-amping is the only sensible option. e.g. Not many people would employ a tube amp for their left channel and a solid-state amp for their right channel.