How does the "Insufficient Funds" error know the amount missing by just passing in let amount?

enum VendingMachineError: ErrorType {
    case InvalidSelection
    case OutOfStock
    case InsufficientFunds(required: Double)   //<<<< here we say that this case has an associate value
}

func vend(selection: VendingSelection, quantity: Double) throws {
        //checking if the seleciotn is correct
        guard var item = inventory[selection] else {
            throw VendingMachineError.InvalidSelection
        }
        //checking if there are items left in the stock
        guard item.quantity > 0 else {
            throw VendingMachineError.OutOfStock
        }

        item.quantity -= quantity   //decreasing the quantity
        inventory.updateValue(item, forKey: selection)  //updating the item

        let totalPrice = item.price * quantity
        if amountDeposited >= totalPrice {
            amountDeposited -= totalPrice
        } else { //If we do not have enough money  (<<< THIS is the part you have to understand)
            let amountRequired = totalPrice - amountDeposited //we calculate how much we need
            throw VendingMachineError.InsufficientFunds(required: amountRequired) //than we throw the error with the associate value
        }
    }

@IBAction func purchase() {
        if let currentSelection = currentSelection {
            do {
                try vendingMachine.vend(currentSelection, quantity: quantity)
                updateBalanceLabel()
            } catch VendingMachineError.OutOfStock {
                showAlert("Out of Stock")
            } catch VendingMachineError.InvalidSelection {
                showAlert("Invalid Selection")
            //we save the associate value in the amount constant, the associate value is calculated in the vend func
            } catch VendingMachineError.InsufficientFunds(required: let amount) { 
                showAlert("Insufficient Funds", message: "Additional $\(amount) needed to complete the transition")
            } catch let error {
                fatalError("\(error)")
            }

        } else {
            //FIXME: Alert user no selection
        }
    }