Sustainable output is what an economy actually gains once its foundations are counted. Ordinary output counts an emptied aquifer as income, a cleanup as growth, and a collapsing fishery not at all until it stops producing. Sustainable output subtracts what was drawn down and what was damaged, and does not count repair as new output.
It matters because it is the figure new money follows. An issuer following ordinary output would add money fastest when the economy consumes itself fastest. Several independent bodies estimate it, the median is used, and physical accounts in tons, hectares, and water volumes are published beside the money values. Where use crosses a line that cannot be undone, the output drawn from it adds no new money at all. See ecological accounting.