Revenue minus costs is taxable profit
Start with everything clients paid you, subtract business expenses and then the other deductions you enter, and you have taxable profit. That subtraction is floored at zero, so a year where costs outrun revenue reports no taxable profit rather than a negative base for the two taxes that follow.
The self-employment tax and the state tax are both computed on this profit, not on the revenue in the bank. Two freelancers billing the same $120,000 can owe very different amounts because one has $5,000 of deductible costs and the other has $45,000, which is why the page keeps the two cost fields separate from the revenue figure.
Other deductions cover retirement contributions and health-insurance premiums - lines that reduce taxable profit before any payroll-style tax lands. The page does not judge whether a particular expense is allowed; it takes the figure you enter and prints the chain, so the arithmetic stays inspectable instead of hiding inside a single combined number.