Quick Answer: The function slider is used for altering a chord's function, will in most instances cause a distinct change in the perceived impact of a harmonic progression.
Function slider and settings I, IV and VWhen opening a project in Liquid Notes, it performs a harmonic analysis of your MIDI arrangement. This enables the software to determine chords, correlation between notes, etc. - the information derived from the harmonic analysis is then presented in the user interface, using vertical bars with two rotary knobs (Chords, Tension) and a slider (Function) for each chord.
The chord function can be chosen by moving the slider up and down to the respective function position (I, IV or V) or by directly clicking on such a position. Changing the function alters the chord's function, and results in an entirely new set of possible chord substitutions. Functions influence the "deep nature" of chord progressions.
I stands for TONIC (first scale degree), IV for SUBDOMINANT (fourth scale degree), and V for DOMINANT (fifth scale degree), the prime steps of a basic cadence. So the chord substitution sequences change from tonic: C...Am... to subdominant: F...Dm... and dominant: G...Em...