A polarizer is a module that can attenuate positively and negatively. Negative attenuation means that the input is inverted. This is some times called an attenuverter.
The S921 has four separate polarizer sections. Each section has an out and a +pol output. The +pol simply adds the knob amount to the out. The purpose of the +pol signal is to provide an offset output so that it’s either always negative or always positive*.
If section 1 (or section 3) is not connected to an input, the knob will act as an offset generator for the next section (sections 2 or 4).
A Mix output is provided for all outs. A Pmix for all +pol outs. And a 1+2 output for out1+out2. And a 3+4 output for out3+out4.
The S921 is mainly meant for processing CV. But it can also be used for audio with no disadvantage. The only thing you may notice is that the knobs are linear not exponential, which can be good or bad depending on what your doing.
*This is assuming the input is always between -1V to +1V