1) The value determined by 'auto'   is dynamic.
2) Optimistic tries to keep audio load below 75% , pessimistic tries to keep it below 50%.

It actually works quite well. In my case as I opted for optimisation of low latency as the prime criteria and quality of sound as the second and 'audio optimistic' was the ideal parameter given the laptop I use ( mac book pro late 2013 i5 2.4 Ghz 4 cores)