The Error Trap field is used to specify where PHP error blocks will be displayed on the finished form page. By default all validation with the exception of the captcha block is handled via Javascript, however, if a user came into the form with Javascript disabled they would bypass this validation.

As protection against this, RackForms validates all fields on the server side, the errors this validation returns are displayed in the Error Trap field element if used.

For reference, if no Error Trap elelement is provided, Sortable pages have errors appended to the bottom of the last field element, Free Form Pages have them appended to the bottom of the RackForms-output-div div html element.