In LLE physical processes, there is this level (interface) between the 2 components (for example) that is being separated as shown in the picture. I know that the height of this level from the reference point (0%) tells us the efficiency of separation. I don't understand the mechanism of what this inferface level means and how does it tells us the efficiency of the process. What does it tells us if it is more or if it is less than the opitmal level (given by engineers) ?