What is a specific process rule?

A specific process rule says a particular step has to happen in the trade area. You see them most for textiles, where the rule can reach back to spinning the yarn, and for chemicals, where a chemical reaction can be enough on its own.

An example

Most clothing under USMCA follows what's called a yarn-forward rule. The yarn has to be spun in the region, the fabric made there and the garment cut and sewn there. A shirt sewn in Mexico from fabric woven in Asia doesn't qualify, however much work went into it.

Chemicals

Many agreements let certain processes stand in for a tariff shift in the chemical chapters, such as a chemical reaction or purification done in the area. The product can then qualify even when its code doesn't change.