The exposure remaining once the planned response has been applied, scored on the same scale as the inherent assessment so the two can be subtracted.
Residual risk is what is left after the response, and it only means anything next to the inherent score it was derived from. The gap between the two is the measurable value of the response, which is the figure a sponsor is really being asked to fund. A residual score equal to its inherent score says the response has bought nothing yet, and that is a signal rather than a rounding error. A residual score higher than its inherent score is possible and worth recording, because a response can introduce exposure of its own.
Where this comes up
One bar per variable, the widest at the top, all of them measured against a baseline down the middle. A tornado diagram ranks what could move an outcome, and by how much.
A risk might happen. An issue already has. That single difference decides which artefact a line belongs in, what you record about it, and who you have to tell.
Contingency reserve sits inside the cost baseline, for risks already named and priced. Management reserve sits above it, for the ones nobody named. Level and authority, not size.
Most risk registers record one number per row. The number that actually justifies spending money is the distance between two of them.