Functional unit
The quantity of product a footprint is expressed per — one garment, one tonne, one square metre, one kWh delivered.
The functional unit answers “per what”. Without it a footprint is a number without a denominator.
It should describe a function, not just a mass, wherever the two differ. Comparing two insulation materials per kilogram is misleading if they need different thicknesses to do the same job; the honest unit is per square metre at equal thermal performance.
Choosing it is the first decision in an assessment and it constrains everything after, which is why two footprints with different functional units cannot be compared however carefully each was made.