Context - This is a proposal from Matchory; we build a comprehensive database of manufacturing companies, their customers, and shipments between them. Currently, we have a database of around 10 million companies and about 800 million individual shipments, all structured as an extended schema.org vocabulary, stored as JSON-LD.
While designing the data structure, we had to come up with several additional properties; some of them are too specific to B2B shipping for schema.org (we plan to publish our own vocabulary in the future), but others would make good candidates for inclusion. This is one of them.

Proposal - Schema.org already provides a lot of properties for shipments and trading (#3122 being highly anticipated); we would like to propose adding a property for the customs declaration code from the internationally used Harmonized System. More than 200 countries are members in the issuing organisation and use those codes for imported and exported goods. Every merchant buying and every supplier selling goods knows, and searches for, those codes. Every participating country requires these codes to be declared for goods leaving and entering customs.

An HS code is a hierarchical, numeric string, consisting of three two-digit groups of wares, from less to more specific. This makes them ideal for broadly categorising products. As HS is an internationally used and maintained standard, it fosters interoperability across vendors and platforms.

Therefore, we'd like to propose to add a new hsCode property to the Demand and Offer classes. This would fit in well with the existing gtin property, but provide a way to express offers or demand for a category of products, not just a single instance.

Alternatives - Currently, we use the category property to store the HS code, but that is only available on Offer, not Demand, and it doesn't feel particularly correct to store the customs classification code in a general product category.

It's our -- my -- first time contributing to schema.org, so apologies for any missteps. I'm more than happy to carry this forward, though.