This walkthrough from Mastering.com breaks down Logic Pro's Flex Time algorithms and when to use each one, using a comped vocal take as the example.
Monophonic suits single-note sources like vocals, bass, or lead guitar. Polyphonic handles chords from guitars, piano, or pads, while Rhythmic works best on rhythmic melodic parts, sometimes interchangeably with Polyphonic depending on what sounds most transparent.
Slicing is built for drums. It moves transients instead of stretching audio, so you can tighten timing to the grid without smearing attack or tone.
Speed and Tempophone are the creative outliers, built for obvious time-manipulation effects like slowing, speeding up, or adding a mechanical tape-like character rather than transparent correction.