The safety argumentation assumes that random errors in the underlying OPC UA stack including its communication links are not too frequent, i.e., that its failure rate is lower than a given threshold, depending on the desired SIL (see Clause 11.3.1).
Whenever the SafetyConsumer detects a faulty message, it checks whether the assumption is still valid, and switches to fail-safe substitute values otherwise. Returning to process values then requires an operator acknowledgment.
Operator Acknowledge is expected to be initiated by a human operator who is responsible to check the installation, see Table 40, row “Operator Acknowledge”. For this reason, the parameter OperatorAckRequested is delivered by the SafetyConsumer to the safety application. See Clause B.2 for details on operator acknowledgment scenarios.
Timeout errors do only require an operator acknowledgment if operator acknowledgment is required by the safety function itself. In this case, SafetyOperatorAckNecessary is set to indicate that operator acknowledgments are required.