The pitch frequency measures and describes what a beat sounds like, i.e. does the isochronic beat have a deeper bass sound, or is it high pitched and sharper sounding? The beat waveform frequency describes how many times the beat is repeating per second, i.e. how fast it is beating. You don’t really need to concern yourself about the pitch frequency, as that doesn’t have a direct influence on brainwave entrainment and doesn’t play a part on the measurement on an EEG. I only mentioned it because you were referring to humans not being able to hear below 20Hz. I change the pitch frequency just to suit the mood of the track. For tracks that are to be relaxing, for meditation or sleep, I tend to use a lower pitch frequency so it sounds deeper and more relaxing and I never have that lower than 100Hz. For an energizing, high focus track I might use a more higher pitched 200Hz isochronic tone, because that is sharper sounding and less likely to make you feel sleepy. That’s all you really need to know about the pitch frequency.
Neural oscillations are rhythmic or repetitive electrochemical activity in the brain and central nervous system. Such oscillations can be characterized by their frequency, amplitude and phase. Neural tissue can generate oscillatory activity driven by mechanisms within individual neurons, as well as by interactions between them. They may also adjust frequency to synchronize with the periodic vibration of external acoustic or visual stimuli.