The capo is a simple device that clamps onto the neck of the guitar. As its use is instructed in the book, its purpose is to change the overall key of the guitar.
Though instructions for using the capo given in the book are for accompanying fiddle tunes, the capo is equally useful to singers. The reasons for using the capo differ for vocal and fiddle music. For singers, the reason for changing the key is to make various songs better fit the singer's vocal range. By using the capo, nearly any song can be made to fit nearly any voice. For fiddle accompaniment, the reason for using the capo is that some fiddle pieces are set in keys that call for difficult guitar chords. The capo allows these pieces to be accompanied by playing only easy chords.
In every instance where the use of the capo is prescribed in the book, specific chord conversions are given at the score. Below is a comprehensive conversion chart, and below that are a couple of visual examples that may prove helpful. The chart covers all chords used in the book and goes to the 11th fret. The 12th fret is the octave, at which point the letter names are the same as for the open strings (no capo,) and the conversion series given in the table starts over, the 13th fret being the equivalent of the 1st fret, and so on.
However, this is only theoretical. In use, the practicality of capo placement at higher-numbered frets becomes questionable. For one, fhe fingerings of the chords become increasingly crowded as the spaces between the frets get smaller. For another, the some guitars have a neck that joins the body at the 12th fret of the fretboard. Depending on the design of the neck, it may be physically impossible to use the capo beyond the 9th or 10th fret. In any case, if using the capo at a high-numbered fret is considered, individual boundaries of sensibility will need to be explored.
Below you can see and hear a couple of examples showing capo placement and the resulting change of key and pitch. Note that placing the capo "at" a certain fret means placing it in the space between that fret and the next lower-numbered fret. Functionally, this makes the designated fret become the nut.
In the examples, a bit is played without, then with the capo. In one, the D chord becomes an F chord. In the other, the C chord becomes a G chord.