Forces the signing party to select a reason from the list when signing the form. If the Signing Reason list is not required, the signing party can use any other signing reasons that may be displayed in the Certify Document or Sign Document dialog, or they can enter another reason. If the Signing Reason list is required, the signing party must select one of these signing reasons; they cannot add their own reason. If the Signing Reason list is required but no signing reasons are added to the list, Acrobat 8.0 and later disables the Reasons list.
See also 

Signing Reasons tab (Signature Settings dialog box)