Measurement
Allometric equation
An allometric equation estimates the mass of a tree from things that can be measured without cutting it down, usually trunk diameter, height and wood density. It is how a forest inventory turns tape measurements into tonnes of biomass, and from there into carbon.
Equations are built by felling and weighing sample trees, so each one holds for the species, region and size range it was built on. Applied outside that range, it can be badly wrong in either direction.
Choosing the equation is therefore a decision to record and justify, not a default. Two teams measuring the same trees with different equations will report different carbon.