What is cumulation in rules of origin?

Cumulation means materials from a partner country count as if they were your own. Without it, a component from your neighbour across the border would be just another imported material.

An example

A US company makes jackets from fabric bought in Mexico. If that fabric qualifies as originating in Mexico, USMCA treats it as originating in the US too, so it doesn't have to meet the jacket's rule.

How far it reaches

The basic version covers the two partners. Diagonal cumulation goes further and takes in other countries that use the same rules, which is how the Pan-Euro-Mediterranean system works. Full cumulation goes furthest and also counts work done in a partner country, even when that work didn't make the material originating on its own.

The catch

You need proof the partner's material really does originate, usually a declaration from the supplier. Diagonal cumulation also only works when the right agreements between all the countries are in force.

Where you'll find it. The article it's in, agreement by agreement.