The number of times that the port was unable to transmit frames because the transmit BB credit was zero. The purpose of this statistic is to detect congestion or a device affected by latency. This parameter is sampled at intervals of 2.5 microseconds, and the counter is incremented if the condition is true. Each sample represents 2.5 microseconds of time with zero Tx BB Credit. An increment of this counter means that the frames could not be sent to the attached device for 2.5 microseconds, indicating degraded performance.

2.5 microseconds * 400 000 = one minute
if you hit the 400k mark your link is bottlenecked and has no possibility to transmit frames.

XorMon NG presents time in % when the port was unable to transmit frames.
100% means that the port has no possibility to send anything for the given time frame.
Switch and Total aggregated graphs make a summary of all per port % then the result can go above 100% what does not have to be wrong if you have many ports.