B

• base leg

  ATC

• beginning of precipitation (time in minutes) (weather reports only)

  Weather

• Bering Standard Time (time groups only)

  Weather

• blue

  General

• center clearance delivered

  ATC

B has 5 meanings; the right one depends on context. A weather report, a NOTAM or an ATC clearance. Got a NOTAM full of contractions? Paste it into the decoder.

Source: FAA Order JO 7340.2 Contractions. Not for operational use.