Sorry if this is an obvious question, but I just want to clear this up. As far as I understand about voice leading, there are essentially two main types. Homophony and polyphony. Functional harmony seems to fit homophony 100 percent since it’s all based on the chord progressions and tones within the chords. Am I understanding this correctly? Can you use functional harmony in a polyphonic type of way?
You sure can. Imagine you composed a tune. Imagine you composed another tune. Imagine you wanted to play those two (or several tunes) together, simultaneously. In order for this combined entity not to sound like a train wreck you would need to use your knowledge of functional harmony to ensure that at each and every given moment the notes from these two, three, four or eleventeen tunes combine appropriately (harmonise). One of the attractions of polyphony is the tension-release offered when there is momentary dissonance between voices, but even this can be analysed via functional harmony.