Reasonable compensation
The salary an S corp has to pay an owner who works in the business. It should match what the work would cost from someone else.
Reasonable compensation is the wage an S corp has to pay a shareholder who works in the business before it pays them distributions. The IRS says on its S corporation compensation page that "S corporations must pay reasonable compensation to a shareholder-employee."
There is no set percentage. IRS fact sheet FS-2008-25 lists factors courts have weighed: training and experience, duties, time spent, dividend history, pay to non-shareholder employees, how bonuses are paid, what comparable businesses pay, compensation agreements and the use of a formula. The IRS also looks at where the company's gross receipts come from. Money earned by the owner's own work should mostly come out as wages.
Salary carries Social Security and Medicare tax. Distributions don't. So a low salary saves tax, and an unreasonably low one can be reclassified by the IRS as wages, with back taxes and penalties.
StructureCost uses 40% of profit, at least $30,000, as a default so the comparison has a number. It is a starting point, not IRS guidance.