When the process is operating at Point A where the graph has a positive slope, departures from the set point tend to be self-correcting. When the substrate concentration goes up, the specific growth rate coefficient also goes up to consume substrate more rapidly. When the substrate concentration goes down, growth rate also goes down so that substrate is used up more slowly. In other words, departures from the set point result in changes that minimize the effects and help to correct the upsets.
The situation is just the opposite at Point B where the graph has a negative slope. Now an increase in S causes a decrease in specific growth rate coefficient to reduce the rate of consumption of S. A decrease in S affects growth rate so that consumption rate of S is greater. This means that a small departure from the set point in either direction makes things worse. We see that the process is inherently unstable at Point B, and designing a good controller, while not impossible in any sense, is relatively much more difficult than for operation at Point A.