How Minitab chooses centering values for short-term statistics for Process Report

Minitab considers three possibilities for centering values in calculating short-term statistics:

The target, when one is specified
The midpoint of the specification range, (USL + LSL)/2, when both specification limits are given but no target value
The process mean, when only one specification limit is given and no target value is given

The target value does not affect the long-term Z calculations. The long-term Z-values center on the mean of the data.