I'd guess it means they can go 2000 either way using more or less the same footprint and without affecting the rest of the proposals in any significant way. FWIW, I think around 18,000 is about right - it's a capacity we'd be more likely to hit on a more regular basis, and I'd rather a smaller stadium close to full than a bigger one with more empty space.
It could also mean 18,000 all seater, or 20,000 with a standing section.