When quoting a position within such a spectrum, it is necessary to indicate whether the quoted position is the USB position or the corresponding LSB position. The SideBand attribute provides this indication. Another option that the SideBand attribute provides is to represent a spectral position by its topocentric offset from the LO frequency.
In practice, the LO frequency is specified by giving the distance from the LO frequency to some "central" spectral position. Typically this central position is that of some interesting spectral feature. The distance from this central position to the LO frequency is known as the "intermediate frequency" (IF). The value supplied for IF can be a signed value in order to indicate whether the LO frequency is above or below the central position.
AST A Library for Handling World Coordinate Systems in Astronomy