Marginal costing definition
Marginal costing is defined in the glossary as "The Cost of continuing to provide the IT Service. Marginal Cost does not include investment already made, for example the cost of developing new software and delivering training."
In accountancy terms the marginal cost is typically defined as, for the sake of simplicity, "the cost of doing one more thing (unit of work/activity)", for example how much does it cost to extend service hours by one hour.
The definition as given sounds more like standard costing ( which isn't in the glossary )