Net Operating Income (NOI) , as one put it, is the center of the universe. Practically all valuations are based on NOI. It is the Net income from the property as if there are no Mortgages.
A couple of definitions first:
Gross Scheduled Income (GSI) is the income expected from the rental property assuming fully rented + any other income, say laundry.
Gross Operating Income (GOI), as the name implies, is the funds you have to operate the property. It is the GSI – Vacancy allowance.
NOI = GOI – Operating Expenses
Cash Flow Before Tax(CFBT) = NOI – Debt Service (principal and interest)
A four-plex renting $850 a unit, expecting 2 months/year vacancies, and $4,000 annual operating expenses.
GSI = 850x4x12 = 40,800
GOI = 40,800-(850x4x2)=34,000