If the custom field is present in all required backend forms, the field is listed with a “+” and can be added to the view. If however, it is missing on any of the required backend forms, the field is listed with a "!", and cannot be added to the view. A short description for the "!" is provided at the bottom of the screen.
When you add fields to a view, all the fields are displayed vertically. If you add a large number of custom fields, only 5 fields are displayed on the view, and the rest are hidden. You can click the Show More link to view all the fields.