Six-four (second inversion) harmonies
In the example below, the Bb and the G in the final bar function as neighbor notes onto the dominant. As discussed in the handbook, six four (or second inversion) chords such as this are best understood not as a harmony in their own right but as a decoration of the root position chord onto which they resolve. The six four is similar to an appoggiatura or a suspension in that it is ultimately understood as a linear rather than a harmonic phenomenon.
This does not mean, however, that six-four chords cannot be elaborated in the foreground. It is quite common for a second inversion chord to support a linear progression or an arpeggiation on the surface of the music, but such an elaboration this will almost certainly still be understood later in the analysis as subordinate to the harmony onto which the six four resolves.