“Regular” fields are simply the Klaviyo predefined fields such as email, phone_number, first_name, last_name etc. These are mostly the basic details. You will need to configure at least an email in the regular fields section in most cases.
You can simply use the “Add Field” button in the map fields section to dynamically add new data to the payload sent to Klaviyo. Inversely, you can use the remove button beside every mapped field to dynamically remove data from that payload.
Note: Remember to save your form after configuration.