-B n   Maximum VBR/ABR bitrate

Specifies the maximum allowed bitrate when using VBR/ABR. Please see -b for restrictions on the value of n.

The use of -B is not recommended. A 128 kbps CBR bitstream, because of the bit reservoir, can actually have frames which use as many bits as a 320 kbps frame. VBR modes minimise the use of the bit reservoir, and thus need to allow 320 kbps frames to get the same flexibility as CBR streams.