From the article, here is the description of the current JCT and its role (footnotes omitted):
The JCT is a bipartisan committee of ten members of the House and Senate tax‐writing committees, and exists principally to provide justification for its staff. The committee does not report legislation, and rarely convenes hearings or performs other traditional functions of a legislative committee. The staff of the JCT — currently including about 50 economists, lawyers, and accountants — assists every member of Congress at each stage of the tax legislative process, and provides a source of tax expertise that is independent of the executive branch. The staff is nonpartisan rather than bipartisan; unlike staff supporting most other Congressional committees (including certain joint committees), the JCT staff is not affiliated with any party and is not separated into majority and minority party staff members.
Although the staff serves all of Congress, its principal duty is to be a policy advisor to the chairs, ranking members, and other members of the tax‐writing committees. In this role, the staff helps to develop, analyze, and evaluate many tax policy options for those committees and assists with all of the legislative tasks necessary for enactment of a bill. In addition, the staff provides the official revenue estimates used by Congress for all proposed tax legislation. The staff also reviews all tax refunds in excess of $2 million and monitors the administration of the tax laws by the IRS. Occasionally, the staff performs tax‐related investigations, such as examining President Nixon’s tax returns and the tax positions of the Enron Corp. The JCT and its chief of staff are given direct access to otherwise confidential tax return information and permitted to delegate that access to others.Here is the abstract of the article: