[KEYS] button: This specifies whether the constituent notes of the arpeggio performance will change according to your keyboard playing. For example, if you want the constituent notes that you’ve once specified from the keyboard to remain the same while you add additional notes from your keyboard performance, turn the [KEYS] button off.
[BEAT] button: This specifies whether your performance will affect the arpeggio pattern. For example, if you want to change the chord without changing the performance pattern, turn the [BEAT] button off and the [KEYS] button on.
If KEYS and BEAT are both off, the current arpeggio performance is kept. You can freely solo on the keyboard while that accompaniment remains fixed.