The band edge generally refers to the lowest (or highest) frequency in the band where one can legally operate within the limits of his/her license. For U.S. operators, for example, this would be approximately 21200, say, for 15 (USB), but would be about 7127 or so for 40 (LSB). The reverse is true for the upper band edge–e.g., on USB on 15 meters the practical limit for transmit frequency is approximately 21448. The lower band edge is sometimes desirable for U.S. operators who are running since some bands inside and outside the U.S. align differently.
An Amateur Radio Blog by DX Engineering 
