The center line for the Xbar chart represents the average of the plotted points (also called the process mean).
The center line on the S chart is the average of all subgroup standard deviations.
Minitab Express displays the mean in the output pane and indicates whether the value is estimated from the data or is specified.
Use the center line to observe how the process performs compared to the average. If the process is in control, then the points vary randomly around the center line.
Do not confuse the center line with the target value for your process. The target is your desired outcome. The center line is the actual outcome.