Although the decision by Assistant Secretary of State Robert D. Hormats to resign from government primarily reflects a desire to seek new challenges in the private sector, it also reflects differences over economic policy making within the Reagan administration.

Among the major points of difference between Hormats and the administration is his concern about the implications of President Reagan's decision last Friday to toughen his stand on the sale of American oil and gas equipment to the Soviet Union. Such equipment would be used to help build a gas pipeline from Siberia to Western Europe.

Instead of easing the restriction--as the U.S. summit partners expected him to do--Reagan extended the sanctions to American subsidiary companies and to foreign companies producing such equipment under American license.

While Hormats had opposed the Soviet gas pipeline, he had argued before the economic summit in Versailles that a negative U.S. decision on equipment sales at this stage would have an adverse impact on the Atlantic Alliance, and would harm American companies. Along with other State Department officials, Hormats contended that extending the ban would discourage foreign firms from purchasing American licenses in the future.

But at last Friday's meeting on the issue, Secretary of Defense Caspar Weinberger, supported by White House aides, reportedly argued that the president had to restate a tough line on East-West economic issues, because the Versailles declaration had been too soft on the question of European credits for the Soviet bloc.

With Secretary of State Alexander Haig absent at the United Nations, White House staffers also claimed that, whatever effect the Versailles declaration on credit might have had, it had been undercut by French President Francois Mitterrand and German officials, who said that they wouldn't change their credit policies toward the Soviet bloc. Although Haig is said to feel that the French-German statements were designed mostly for home consumption, it was decided that, as a matter of principle, Reagan should sound "presidential" on the question.

Hormats had told Haig as far back as last May that he intended to resign later this summer. Hormats has served in high posts in the past four administrations. He has most recently been coordinator within the State Department for foreign economic policies, and also had the chief responsibility of preparing the U.S. position at Versailles.

Hormats also is known to be frustrated by being called on for at least the past six months to do not only his own job and the summit preparations, but to fill the role of the undersecretary of State for economic affairs--without the title. Myer Rashish had resigned from that post early this year, for reasons never plainly stated, after much bickering within the department.

Hormats was Haig's choice to succeed Rashish, with Hormats' current post to be filled by Richard T. McCormack, a former staff member of Sen. Jesse Helms (R-N.C.) Hormats wasn't promoted, and McCormack has continued as a consultant at State and is considered a likely prospect to succeed Hormats.

Insiders suggest that, although Helms--who once opposed Hormats as "too liberal"--withdrew his opposition, Hormats was regarded by White House staffers as "insufficiently conservative."

Hormats is highly regarded on Capitol Hill for having brought a sense of continuity to the problems of trade and international development policy to the executive branch during his 12 years of service in which he played a key role in all eight economic summits.