The Sine Level knob mixes a sine wave (at the frequency of oscillator 1) directly into the dynamic stage, independent of the filters. Even if you have filtered away the basic partial tone of oscillator 1 with a highpass filter, you can reconstitute it with this parameter.
In cases where oscillator 1 is frequency-modulated by oscillator 2 (if you have turned up FM with the waveform selector), only the pure sine wave is mixed into the dynamic section, not the distorted FM waveform.
Any modulation of oscillator 1 pitch set in the router affects the frequency of the sine wave mixed in at this stage.
Note: The Sine Level knob is useful for adding warmth and a fat bass quality to the sound. Extra body can be added to thin sounds with this parameter, given that oscillator 1 actually plays the basic pitch.