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