check boxes: are individual elements and can be preselected or not by config, the purpose is to allow users to opt in or not

check box groups: see above, but the concept supports a family of check boxes (again: still optional)

radio buttons: are always a group of elements, of which only one can be selected (preselection by config). usually one radio button is always preselected but cforms even supports no radio box to be selected.

Exception for check boxes: individual check boxes can be made mandatory (which really does defeat the purpose of a check box, but support concepts such as 'terms of use' etc.)

What you need, is covered by the  'multiple drop down' field.