During the production phase, any change in design that is NOT a Class I Change. A Class II Change is an external change. Contrast with Class I Change.
The primary difference in the execution of a Class I Change vs. the execution of a Class II change is one of permission vs. forgiveness (respectively). If the supplier executes a Class II change with which the customer later non-concurs during Acceptance reconciliation, the supplier might get a wirebrushing, up to and including contractual penalties.