If you would like to create a custom field on the Personal Information screen (or the Add/Edit Contact screen for your contacts) for your registrants, follow this procedure.
(1) Go to Preferences->Personal Information Options scroll to the bottom, and then create the question under "Add New Question". Set "Visible" and "Required" as you need it for your Add/Edit Contact screen. These settings will be used for the Contact screens, and also be the default for any new event and its PI screen.
(2) If you want to change the settings, Visible/Required, in a particular new event, go to the event->Manage Event->Personal Information and change the settings in that event.
(3) If you have existing events that were created before your new custom PI field (and you want to add the new custom PI fields there), go to that event->Manage Event->Personal Information and update the settings to include that new field.