Direct measurements of greenhouse gases emissions are not possible. Assessments are therefore using precise theoretical calculations for each sectors. They are called “emission factors” and are regularly updated. Trying to quantify emissions related to a given activity requires consideration of complete life-cycles. That means counting emissions from all related activities, raw materials, transformed products and necessary infrastructure (indirect emissions). Naturally, one has to stop imputing indirect emissions to the evaluated activity at some point. This will depend on the working branch and on the assessment methodology used.
From collection: Kick the Habit: A UN Guide to Climate Neutrality