The --rd sections signal requires that details be added to a CRM Group Call before submitting it. If a group Call has a nonperson account as the parent, attendees are required in order to add details. There is a configuration that skips adding an attendee to a Call, thus skipping the --rd section signal.
If using the --ao section signal, it allows a user to bypass adding attendees, which allows users to bypass adding a detail to the Call.
CRM Help Documentation: Section Signals