The tariff number of an item, also known as the "harmonized code" or "HS code," is a standardized number given to a particular product or type of product for easier identification during customs processing and better standardization of international shipping. These product-specific codes are documented in the Harmonized System (HS) maintained by the World Customs Organization (WCO).
Tariff numbers are not required for international shipping. Not including a tariff number should not negatively impact your package's shipping progress. However, in some cases, adding one can help smooth the customs process.
Tariff numbers exist for almost every product involved in global commerce and ensure uniformity of product classifications worldwide. A complete tariff code is no less than six digits and can be up to ten. The more digits in a tariff code string, the more specific the product it identifies. A string of less than six digits is considered a partial tariff number and represents a broad product category or chapter of products in the HS, rather than a single item.
When creating an international label in Shippo, you can add a tariff number to a customs item on Step 1.1 of 3, which is the Customs Declaration and Customs Items. Either add a new customs item, or click "edit" next to an existing customs item, to add this information.
There is no way to add default customs items/tariff numbers in Shippo, so if you do use them, you will have to add them manually during this customs step of the label creation process.
You can find more details and the full list of tariff numbers/harmonized codes here: https://www.usitc.gov/tata/hts/index.htm
You can also search the full World Customs Organization HS code database here, but this does require a paid subscription: http://www.wcoomd.org/en/topics/nomenclature/instrument-and-tools/hs-online.aspx