Goods crossing a border are taxed according to where they came from, and that answer is rarely obvious. Origin rules exist because modern products are made from parts sourced across many countries.
Origin is a legal test, not a location
A phone might contain components from a dozen countries and be assembled in a thirteenth. Customs still needs one answer for the tariff line.
Rules of origin supply that answer through a defined test, applied consistently rather than by asking where the factory happens to stand.
The result can differ from ordinary intuition, which is why two visually identical goods can attract different duty rates at the same border.
The two standard tests
The first asks whether the goods were wholly obtained in one country. Minerals mined there and crops grown there qualify without further analysis.
The second applies to everything made from imported inputs and asks whether those inputs were substantially transformed by the work done in the exporting country.
Substantial transformation is usually measured either by a change in tariff classification or by a minimum share of value added locally.
Why the tariff-classification test is common
Every traded good carries a classification code, and the code for a finished product differs from the codes for its parts.
If the inputs enter under one heading and leave under another, a real manufacturing step has occurred, and the test is satisfied without valuing anything.
That makes the test comparatively cheap to administer, since a customs officer compares codes rather than auditing a factory's costs.
Value thresholds catch the marginal cases
Some processes change a code without changing much else, so agreements often add a requirement that a set proportion of value be added locally.
Meeting that threshold means opening the books, because the calculation rests on the cost of imported materials against the ex-works price.
Exporters therefore keep detailed bills of materials, and a change of supplier can quietly move a product in or out of preferential treatment.
What origin rules are actually for
Trade agreements lower tariffs between their members, and without origin rules any outside exporter could route goods through a member state to claim the same benefit.
The rules make that unprofitable by requiring genuine production inside the area rather than a change of address on the paperwork.
They are, in effect, the enforcement machinery of every preferential agreement, which is why negotiating them absorbs so much of the drafting time.