Cal. Lab. Code § 5505
If any defendant desires to disclaim any interest in the subject matter of the claim in controversy, or considers that the application is in any respect inaccurate or incomplete, or desires to bring any fact, paper, or document to the attention of the appeals board as a defense to the claim or otherwise, he may, within 10 days after the service of the application upon him, file with or mail to the appeals board his answer in such form as the appeals board may prescribe, setting forth the particulars in which the application is inaccurate or incomplete, and the facts upon which he intends to rely. A copy of the answer shall be forthwith served upon all adverse parties. Evidence upon matters not pleaded by answer shall be allowed only upon the terms and conditions imposed by the appeals board or referee holding the hearing.
Ca. Lab. Code § 5505