Predefined fields are useful for when you wish to add lots of similar fields to an event type.
When constructing a form within an event type section you will be adding fields of varying types to this form.
In lots of cases custom fields will be added to forms and often it is necessary to add many similar fields across the same, or different forms. When choosing to add custom field you are first presented with a list of all the custom fields which have previously been saved as predefined fields.
This list allows you to quickly select a previously saved field and add this to the form. This can be especially useful when building discrete lists in order that the options do not need to be entered repeatedly.
To save a new predefined field simply add a custom field and then select save this field as a predefined field from the settings of that field during creation. This will add this field to the list of predefined fields.