Optional extras can be added to a booking at booking time.
The name and description are obvious, as is the price.
The quantity option allows you to configure the maximum number of the extra that can be chosen at booking time. If the quantity is greater than 1, then a dropdown appears in the booking form to allow the guest to choose a quantity.
The Automatically Selected option, if set to yes, means that this extra will automatically be set to selected in the booking form when the booking form opens, and differs from the Force option in that the guest can de-select if if they want.
Below that, you can choose the extras model, which can apply multipliers depending on what you've chosen. The 'force' option allows you to ensure that it is always added to a booking, because it cannot be removed. You might use this option if you want to itemise something like a security payment.
The optional extra image can be uploaded using the Media Centre feature.