Invisible and Hidden Fields in Forms 

When you sign or certify a form, you also sign any invisible or hidden fields. These fields are handled in the same way as visible form fields. If any subsequent changes are made to these fields, you are shown an information message when the signature or certification is checked. A certification is invalidated if changes to form fields are not permitted. For more information about setting which changes are allowed, see Certification of Forms.

You create invisible or hidden fields in Adobe LiveCycle Designer. For more information, see the online help for this tool.