Number of Digits
The longer the string of digits, the greater the degree of specificity of the commodity. One or two digit strings represent greatly aggregated data for broad categories of commodities. Seven or ten digit strings represent fairly specific commodities.
Systems in Use
The two main systems in use today for international trade statistics are the Harmonised System (HS) and the Standard International Trade Classification (SITC). Before the United States adopted HS in 1989, some of its published data was organized according to the Tariff Schedules of the United States of America (TSUSA), and really early data may just be organized under verbal headings..
Be aware of the revision being used for the data that you're studying and use the appropriate revision to look up the code.
Indexes and Keyword Searching
Although you can get a code number quickly from a keyword search or from an alphabetic index in the printed code books, the full definitions will give you a better idea about how that commodity is being defined and show its context to other commodities.
In electronic sources, you can generally search for matching text to find commodities or else drill down by expanding a numerical hierarchy. Printed guides will have tables of contents for browsing or indexes for word lookup.
The latest version of the Harmonized System (HS) adopted by the United States is published by the U.S. International Trade Commission (ITC) as the Harmonized Tariff Schedule of the United States. Goes to the 10-digit level of detail.
Published by the United Nations as the Commodity Indexes for the Standard International Trade Classification. Usually not quite as detailed as the Harmonised System. Goes to the 5-digit level; some data sources are more detailed.
Paper (in Perkins/Bostock Library)
Used by the United States before its adoption of the Harmonized Schedules. Data through 1988 was disseminated using both SITC and TSUSA categorizations. You must refer to the following book to use any source categorized with TSUSA. Once you have a Harmonized Class, use the second half of the following book to cross reference from the 8-digit Harmonized class to a 7-digit TSUSA class. This is only approximate, but you can figure out the context when you look in the data sources.
Paper: Continuity of Import and Export Trade Statistics After Implementation of the Harmonized Commodity Description and Coding Systems. Perkins/Bostock Reference U.S.: ITC 1.12: 332-250