Personal Data Protection 29 (3) Where a code of practice is registered by the Commissioner under section 23 or 24, the Commissioner shall notify, in such manner as he may determine, the relevant class of data users to whom the code of practice is applicable— (a) of the identity of the code of practice concerned and the date on which the code of practice is to take effect; and (b) of the specific requirements under this Act for which the code of practice is issued and registered. (4) If there is any uncertainty or ambiguity as to which code of practice is applicable to a particular data user or class of data users, the data user or person concerned may apply to the Commissioner for his opinion on which code of practice is the applicable code of practice in relation to the circumstances of such data user or person. (5) The Commissioner shall provide his opinion within thirty days from the date of receipt of an application made under subsection (4). (6) The Commissioner shall, when making his opinion under subsection (5), take into account any relevant previous opinions, if any. (7) The Commissioner may withdraw an opinion made under this section if the Commissioner is satisfied that the nature of the activity engaged by the data user has changed materially. Revocation, etc., of code of practice 26. (1) The Commissioner may revoke, amend or revise, whether in whole or in part, any code of practice registered under this Act— (a) on his own accord; or (b) upon an application by the data user forum or such bodies representing the data users.