The Fuel Correction table sits inside the Water Injection Advanced function in NSP, alongside an Ignition Correction table. Haltech's own help text for this item hasn't been released yet, so we won't guess at exact axes or units here. Check what the table shows in your version of NSP.
The general idea is well established. Injecting water, or a water-methanol mix, cools the intake charge and the combustion chamber. That changes what the engine needs. Methanol is itself a fuel, so a water-methanol mix adds fuel the ECU didn't meter, and the mixture may need leaning slightly to compensate. Pure water adds no fuel but changes combustion. A fuel correction tied to the water injection system gives you a place to make that adjustment only while injection is actually happening.
How we approach it: start with no correction, confirm the system is spraying when it should, and let your wideband logs tell you whether fueling needs to change while injecting. Make small changes and recheck. The wiring side is covered in Water Injection Advanced Wiring. Unsure how to set yours up? Ask us.
