At one time, I could have sworn the options were guitar, bass, and keyboard. I am sure Eventide will explain. The simple version is if the source instrument is a bass, the PitchFactor algorithms are optimized to handle the tracking of the lower frequencies. If your source instrument is not a bass, then it is a “guitar.” So, if you are using a microphone to capture the signal of a low-frequency instrument, use “bass,” otherwise use “guitar.” If I am wrong, I am sure Eventide will explain.