The treble clef displays the signature key that would be assigned to the current chord with the  button.  For example, this key setting:

would assign to the current chord the key of F Major / B Minor.  See the comment below about signature keys.

The up and down buttons are used to change the signature key.  They have no effect until the button is pressed.

The button sets the signature key of the current chord, of a range of chords, or of the entire song.

if no selected range exists (see Selecting a Range of Chords), then the signature key of the current chord is set.

if a selected range exists, then the signature key of all the chords in the range is set.

if the range encompass all chords of the song, then it follows that chord keys are set.

Signature keys have no effect on how chords sound.  FastChords assigns a default signature of C Major to new songs.  However it is best to assign the correct key to songs (based on what the written music says) because they have a direct impact on the transpose operation, and on which keyboard keys are selected when entering melody notes.

A chord signature key is displayed in the chords table by the number an type (e.g. flat or sharp) of alterations it has:

Key

Scale

Shown as

Cb Major / Ab minor

7b

Gb Major / Eb minor

6b

Db Major / Bb minor

5b

Ab Major / F minor

4b

Eb Major / C minor

3b

Bb Major / G minor

2b

F Major / D minor

1b

C Major / A minor

CM

G Major / E minor

1#

D Major / B minor

2#

A Major / F# minor

3#

E Major / C# minor

4#

B Major / G# minor

5#

F# Major / D# minor

6#

C# Major / A# minor

7#

The menu option Selection > Clear Signature resets the signature key of all chords to CM (C major)