When a certain field only needs to show up when another option is selected, for example if you need a field for more info.
To make a field 'conditional', simply click the 'pencil' icon next to its label.
A modal will appear with different field options, of which 'Required' is the second you can select.
Switch 'Required' to 'on'. You will now see the required-specific options appear.
Select Category field: When selected, you have to specify an option from a category field in the form. Hence, a category field needs to be present in the form for this option to be enabled. The field will then only be shown when this specific option from the category field is selected.
E.g. Only when you select the option 'Receive email' from the 'Notifications' category field, would the 'Email' field be required.