HS Codes are harmonised commodity codes used in 176 countries outside of the EU. Customs authorities require orders sent from the EU to non-EU countries to include HS Codes for all the items sold and shipped.
If you want to use HS Codes in the future, make sure to start by activating the Shipping Costs on the Settings. In Odoo, HS Codes are dependant on shipping/delivery operations.
Then, in a Product, under the Inventory tab you will have the HS CODE field (under Logistics subtitle). Unlike Intrastat Codes, the HS Code field is a text field and not a drop-down populated from another model. This means that the Company will need to fill the HS Code field in for every Product. This of course can be Imported, if needed.
Once you have filled the HS Codes on the Product, and once you sell them in an SO, the HS Code will NOT appear in the Invoice (as is common practice that the Invoice with the HS Codes on the Item Lines is sent with the shipment for the Customs Authorities).
Instead, the HS Codes will appear in the Delivery Slip, showing the Items, quantities and their HS Codes (along with the Order number and date). This WH/OUT document (along with the invoice, if necessary) can be printed and must be included with the shipping for the customs authorities.
Hope this helps!