If Attachment Form or Action Form are selected, the UI Designer transposes the case fields of the selected repeating group from multi valued fields to single value fields, because these fields are captured for each document or record. For these fields, you are required to select a Field Group in order to create the form:
Attachment Form: In this form, repeating table controls are not available. Only Single Value case fields and fields from a selected record group are available to design the attachment properties form. At least one repeating field groups that contains at least one repeating field must be added (see Prerequisites section) in order for this option to appear.
Action Form: Includes the action_verb field, which captures the case action being taken. At least one repeating field groups that contains at least one repeating field must be added (see Prerequisites section) in order for this option to appear. If you need to open a case action form from inside of your form, you can use a CallJS rule.
Clicking the Next button proceeds to design mode selection.