Not everybody has an income that is easy to calculate. If someone is self-employed and deducts personal expenses as though they are business expenses (whether such deductions amount to tax fraud or are just aggressive but legitimate tax claims) or earns income from a tax-free or tax-reduced source, you need to convert such circumstances into what that person would be earning in a paradigm case in order to determine child support. Someone who earns $50,000 a year as a teacher, nurse or other salaried employee does not have the same disposable income as someone who earns $50,000 a year tax free (or tax reduced). Since the table used to calculate child support makes assumptions about things like standard tax rates, the income for someone who doesn’t fit the standard model needs to be adjusted or “grossed-up” before that income is plugged into the table that calculates child support. The same thing goes for spousal support.
Sometimes someone’s income is so low that there are questions about whether they are intentionally unemployed or underemployed in order to get out of paying child support (the same thing can be true with spousal support). Courts do not allow parents to do this. In such cases there would have to be an analysis comparing someone’s income with their ability to earn money based on their health, education etc. If a court concludes that someone is intentionally earning less than they could reasonably be earning the court can “impute” income to that person, which means that they can pretend that that person earns X dollars per year and require that he or she pay child support on such an income.
Sometimes there are questions about whether someone is being honest about what his or her income is. There is a whole set of legislation and court cases to deal with the issue of “disclosure” and how to get information to determine whether someone is hiding income (for example by earning cash under the table).