Numbers are not self-explanatory, they only are made of a value, which does not explain their purpose. Moreover, if the same number should be consistently used in multiple places, using a constant will ensure that all occurences are updated at once.
The following code:
netSalary = 0.85m * grossSalary; // Non-Compliant - where does 0.85m come from, what does it mean?
should be refactored into:
const decimal Rate = 0.85m; // Compliant - it's now clear what the 0.85m is netSalary = Rate * grossSalary; // Compliant