Use the Error Label box in the Iterative Alignment dialog box (Insert | Alignment | New | Iterative button) to define a label that the measurement routine flow will go to if the error in matching the measured level, rotate, and origin datum features to their corresponding theoretical features exceeds the fixture tolerance level in the Fixture Tolerance box.
If you supply the minimum number of inputs for each of the datum axes (three for the Level datum, two for the Rotate datum, and one for the Origin datum), PC-DMIS can fit the input feature measurement value to its theoretical values without errors. In this case PC-DMIS doesn't really need the fixture tolerance. If you supply more than the minimum number of inputs for any of the defined datums, then either part or fixture errors may make it impossible to fit the measurement values to the theoretical values with less error then the supplied fixture tolerance.
If you don't define an error label, PC-DMIS will generate an error message displaying the amount of error for each of the datum features, giving you the option to cancel or continue execution with the datums as they are.
To create a label, see "Using Labels" in the "Branching by Using Flow Control" chapter.