"However, the SEA or LEA may not require the use of the model forms.(b) Parents, public agencies, and other parties may use the appropriate model form described in paragraph (a) of this section, or another form or other document, so long as the form or document that is used meets, as appropriate, the content requirements in ??300.508(b) for filing a due process complaint, or the requirements in ??3" . . . . .