An unanswered required question wears the asterisk and holds the submit gate; it never gets an error bubble. The status line above the button tracks what remains.
A radio group is required by required on any member; a checkbox set is required by data-fs-min-selected, which understands the set.
required
data-fs-min-selected
data-fs-required holds a condition. While the condition is true, the field is required, exactly as required makes it. While the condition is false, the field is optional. The field stays on the page in both states. Here the phone number is required only when the person prefers a call. The set of ways to reach them is required only when they ask to be contacted.
data-fs-required
Ready to submit.