A Disclosure Document is an "evidence of conception" of an idea or an invention. In no way does it substitute for a Provisional or a regular utility patent application. On just a fee of $10, it enables the applicant a to have a recorded proof of date of conception provided it is followed up by the regular patent application within two years of receipt of the disclosure document at the USPTO.
Unlike in a Provisional, this date cannot be considered an effective fling date. For this very reason that a Provisional serves the purpose of providing an earlier filing date, it is understandable that as on date, the Disclosure Document option is nearly history.