[Faransky] has created a very impressive Arduino function generator that doesn ... So [Faransky] uses a X9C104P 100KOhm 8-bit digital potentiometer as a voltage divider on the chip’s output.

Among all the Arduino ... function, you won't typically see the value written as a fixed number. Instead, it's periodically changed using two ways. One is using an analog input like a potentiometer.