Customs, shipping and international-trade references — the lookups normally locked inside paid databases.
HS code structure (chapter, heading, subheading), the international 6-digit core, national extensions (HTS, CN), the 21 sections, and the 6 General Rules of Interpretation.
The ICC handbook condensed into one table: who pays freight, who carries risk, where it transfers, and who clears customs. EXW, FCA, FAS, FOB, CFR, CIF, CPT, CIP, DAP, DPU, DDP.