Actually, this comes from international pitch notation. The octaves arranged down from highest to lowest are:
c7 (c=16744.0 Hz, eb7 = 19,912.1 Hz; Human hearing threshold is 20,000 Hz)
c6 (c=8372.0 Hz, b6 = 15804.3 Hz)
c5 (c=4186.0 Hz, b5 = 7902.1 Hz)
c4 (c=2093.0 Hz, b4 = 3951.1 Hz)
c3 (c=1046.5 Hz, b3 = 1975.3 Hz)
c2 (c=523.3 Hz, b2 = 987.8 Hz)
c1 (c=261.6 Hz, a1=440Hz, a1 = 493.9 Hz)
small, or c (c=130.8 Hz = Middle C, b (small b) = 246.9 Hz
Great, or C (C = 65.4 Hz, Great B = 123.5 Hz)
Contra, or CC (CC = 32.7 Hz, BB = 61.7 Hz)
Sub-Contra, or CCC (CCC = 16.4 Hz. Human hearing threshold is 20 Hz +/-, BBB = 30.9 Hz)
See the chart at
http://home1.gte.net/wwalker/Frequency.html for a better layout of this information.
Anyway, "Bb" is the Key. If the lowest "Concert C" your instrument is designed to play is a Sub-Contra C (CCC) and it is a transposing instrument in the key of Bb, it is a BBBb instrument. If it only plays in the Contra (CC) range, it is a BBb instrument. If it only plays in the Great range (C), it is a Bb instrument. If it only plays in the small (c) range, it is a bb instrument (that is small B-flat). If it is a higher pitch, say playing in the c4 range it is technically a bb4 (B-flat 4) instrument. However this is quite confusing, so most manufacturers only state the key such as Bb for a Tenor saxophone.
When you go down low, saying BBBb says you play so low that you can push the bottom edge of human hearing. Saying BBb says you are an octave above that level.
With the average human threshold of 20 Hz, the lowest note you can hear would be EEE or Sub-Contra E. You might hear EEEb at 19.4 Hz but only if you hear low notes better than average. Most people would feel these bass notes rather than hear them.