President Carter, in a decision White House officials contended will make more information available to the public, issued an executive order yesterday overhauling the governments.

The order, the result of a year-long administration review of the existing system, for the first time establishes seven broad categories of information that may be classified if dislosure of the information would cause "identifiable" damage to the national security.

White House officials argued that the changes will result in fewer documents being classified as "top secret," "secret" or "confidential." But the question remained whether the categories in the order are so broad that they will result in no significant change in the classification practices.

"There is no improvement in what can be classified in the place, and there may be more confusion overall," said one congressional staff member who worked with the administration on the order. This aide contended that the categories, in effect, may serve to "legitimize" past classification practices.

But mixed with that criticism was praise for other sections of the order, intended to speed up and generally ease the system for declassifying documents.

"I would be less than candid if I did not say that I am not completely satisfied with this order,"s aid Sen. Joseph R. Biden (D-Del.). "However, I believe the order is a marked improvement over existing policy."

The order reduces the length of time information may be classified, reduces the number of agencies with classification authority, requires that the public interest in disclosure be considered in declassifcation decisions and establishes a new government office to oversee the declassification process.

Six of the categories in the order cover everything from military plans and weapons to "foreign relations or foreign activities of the United States" and "scientific, technological or economic matters relating to the national security."

The seventh, catch-all category covers any other matters "related to national security and which require protection against unauthorized disclosure" in the opinion of the president or other top officials.

Briefing reporters at the White House, Rick Neustadt of the domestic policy staff and Deanne Siemer, the Defense Department's general counsel, said they could not estimate how a reduction in classified information would result from the order.

They also conceded that much will depend on how the new system is administratered by government officials with classification power, each of whom will decide when disclosure of information would cause "identifiable" damage to the national security.

"The president is directing officials to be more restrictive" in the use of their classification authority, Neustadt said.

"On the whole, I think you will find fewer things are classified," he added. "With vigorous oversight, I think it will be substantial."

Under the order, according to the White House officials, most documents will be automatically declassified after six years. Previously, about half the documents classified by the government were declassified after six to 10 years, with the other half remaining classified for 30 years.

The Carter order reduces the 30-year classification period to 20 years. As a result, Neustadt said, the number of pages declassified during the next 10 years will increase from 350 million to about 600 million.

Eleven government agencies that deal primarily with domestic matters will lose their classification authority under the order, and another five agencies will have their power reduced. In most cases, these agencies had little need for an rarely used their power to classify documents, officials said.

The order also requires that most documents be classified on a "section-by-section" basis rather than as a whole.

The new office created by the order, the Information Security Oveersight Office, will be part of the General Services Administration. A congressional staff aide familiar with the drafting of the order said that suggestions that this office be given greater authority by placing it in the White House or executive office of the president were turned down because of Carter's promise to hold down the size of his staff.