Allocation
How the emissions of a process that yields several outputs are divided between them.
A process rarely produces one thing. A refinery yields many products; a dairy yields milk and cream; a smelter yields metal and slag. The emissions have to be split.
ISO 14044 sets a preference order: avoid allocation where you can, by subdividing the process or expanding the system. Where you cannot, allocate on an underlying physical relationship — mass, energy content. Only failing that, use economic value.
The choice moves the result, sometimes a great deal, and it is a choice rather than a measurement. A footprint that does not state its allocation basis has left out something the reader needs.