What it does
The MAP Correction table trims fuel as a percentage against manifold pressure, on top of everything the main fuel table is already doing. How much you should use it comes down to your Tuning Method and your fuel load source.
On a VE tune: start at zero
With Tuning Method set to Volumetric Efficiency, the Nexus already includes manifold pressure in its air mass calculation. Any value in this table becomes an extra correction on top of that. Set the whole table to 0% before you start tuning, and fix fueling in the VE table itself. Using MAP correction to cover VE errors just hides them in a second place. Background in Tuning with VE.
With TPS as the load source
If your fuel table uses throttle position as its load axis, as is common with individual throttle bodies or big cams, the base table doesn't know what manifold pressure is doing. As pressure rises, particularly with boost, or as pumping losses change, the engine's needs move away from what the TPS-based table assumes. That's where MAP Correction earns its place, and it's normal to see real values in it on these setups.
Tune the main table first at a steady pressure, then use MAP Correction for the difference, confirming with a wideband at each step.
See the other fuel corrections in Corrections, including air temp and barometric.
