28 Tex. Admin. Code § 1.42
At the request of any party to a contested matter, or at the direction of the presiding officer, the witnesses may be sworn and removed out of the hearing room to some place where they cannot hear the testimony as delivered by any other witness in the cause. This is termed placing witnesses under the rule. No party to the proceeding shall be placed under the rule. Where a corporation or other legal entity is a party to the proceeding, the presiding officer may exempt from the rule an officer or other representative of such corporation or other legal entity to aid counsel in the presentation of the case. For good cause shown, the presiding officer may exempt other potential witnesses from the operation of this section if the party requesting the exemption states that it will not call any such person as a witness. A request for an exemption precludes the requesting party from calling the exempted person as a witness. If any party be absent, the presiding officer, in his or her discretion, may exempt from the rule a representative of such party. Witnesses, when placed under the rule, shall be instructed by the presiding officer that they are not to converse with each other or with any other person about the proceeding other than the attorneys in the proceeding, except by permission of the presiding officer, and that they are not to read any report of or comment upon the testimony in the proceeding while under the rule.
28 Tex. Admin. Code § 1.42