The big picture
The Department of State, like all bureaucracies, creates a mass of records in the course of carrying out its business. Those records remain in the working files of the department until they are formally transferred to the National Archives. At present, the National Archives holds records up through about thirty years ago. Records created after the mid-1980s are likely still at the Department of State and are not available to researchers. In addition, because many of the records created by the department are classified, they must be declassified before the National Archives can release them to researchers.
First: is what you need in FRUS? Or in one of the declassified documents resources?
1789-1910: records are arranged by city and country until 1906, then according to the "Numerical File" system for 1906-1910. Virtually all of the records are available on microfilm.
1910-1963: records are arranged according to the "Decimal File" system and are available at the National Archives unless classified. There are two main series available in microfilm, arranged by country and date. The microfilm sets are titled "Records of the Department of State Relating to ..." or "Confidential U.S. State Department Central Files" depending on the publisher. See the regional guides for our holdings.
1963-1973: records are arranged according to the "Subject Numeric File" system and are available at the National Archives unless classified. Microfilm/digital publication of material from 1963-1966 has only begun recently.
July 1973 to December 1975: records were entered into the State Archiving System and may be available online through the Access to Archival Databases system, or on paper or microfilm at the National Archives unless classified.
1976 to present: records either have recently been transferred to the National Archives, or are still in the custody of the Department of State, and available only through a Freedom of Information Act request.
Besides the records in the Central File, there are two other major classes of records:
Office Files or "Lot Files" (also in RG 59) -- records of various offices within the Dept. of State, or relating to events, functions or special topics. There is some overlap with the material in the Central File.
Foreign Service Post Files (RG 84) -- records originally held at U.S. consulates, legations or embassies. Much of this material is duplicated in the Central Files.