When is the transaction state set to restored in the updatedTransactions delegate method of SKPaymentTransactionObserver?

I've been testing code for in-app purchases, and I can't get the transaction state to set to restored in the updatedTransactions SKPaymentTransactionObserver delegate method when it's called. When does that method call with that transaction state?

func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {

    for transaction in transactions {
        switch transaction.transactionState {
        // Call the appropriate custom method for the transaction state.
        case SKPaymentTransactionState.purchasing:
            showTransactionAsInProgress(transaction, deferred: false)
        case SKPaymentTransactionState.deferred:
            showTransactionAsInProgress(transaction, deferred: true)
        case SKPaymentTransactionState.failed:
            failedTransaction(transaction)
        case SKPaymentTransactionState.purchased:
            completeTransaction(transaction)
        case SKPaymentTransactionState.restored:
            restoreTransaction(transaction)
        }
    }

}

You get a restored transaction if you call restoreCompletedTransactions on SKPaymentQueue and the user has restorable in-app purchases. See the reference documentation for complete details.