Section 7216 of the Internal Revenue Code, with its regulations at 26 CFR 301.7216, applies to anyone in the business of preparing tax returns or providing services in connection with them. "Tax return information" is broad: it covers anything a client gives you for the purpose of preparing a return, plus what you compute from it.
Some uses are allowed without consent, such as using the information to prepare the return itself. Others, including many disclosures to third parties, require a written consent that meets specific format rules. Penalties can be civil and criminal.
For firms evaluating AI tools, the practical questions are where client data goes, who can access it, whether it is used for anything beyond preparing the return, and whether your consent process covers the tool. Your firm's own policy and legal advice should answer those questions.
General information, not legal or tax advice.