wifi frame "Supported rates" meaning
 

I captured some 802.11 frames with ethereal.
What is the meaning of (B) in "Supported rates"?

IEEE 802.11 wireless LAN management frame
...
Tagged parameters (39 bytes)
Tag Number: 0 (SSID parameter set)
Tag length: 6
Tag interpretation: DUALRF
Tag Number: 1 (Supported Rates)
Tag length: 6
Tag interpretation: Supported rates: 12.0 18.0 24.0 36.0 48.0 54.0(B) [Mbit/sec]
...