I think "subpartida" is referring to the subcategories of merchandise classification on the tariff schedule.

e.g.:
"The importer must determine the classification number of the
merchandise being imported. The Harmonized Tariff Schedule of the
United States (HTSUS), issued by the United States International Trade
Commission, prescribes the classification of merchandise by type of
product; e.g., animal and vegetable products, textile fibers and
textile products. In your case, this would most likely fall under
parent classification of 6103 (with the appropriate subclassification,
depending on material)."
http://answers.google.com/answers/threadview/id/610544.html

"Canada Border Services Agency (CBSA) has also implemented changes to Customs Tariff Item : 3504.00.00, Peptones and their derivatives; other protein substances and their derivatives, not elsewhere specified or included; hide powder, whether or not chromed. Tariff item 3504.00.00 is now separated in 3 subclassifications : 3504.00.11 - Milk protein substances - Within access commitment; 3504.00.12 - Milk protein substances - Over access commitment; and 3504.00.90 - Other."
http://www.international.gc.ca/controls-controles/prod/agri/...