@Jackie Griffin, first we should clarify what 'multifamily' is. By normal (and agency lender) definitions, it is any building or property with 5 or more units. Most of our responses are referring to many more than 5 units and is applicable to any number (hundreds) of units. Larger units need preliminary underwriting to prepare an offer.
(Preliminary) underwriting is normally the very first thing that is done by the purchaser to help screen properties. It helps to put a price on the property that you may be willing to offer based on its potential for earnings.
Most multifamily brokers provide full financials for available properties. This usually (should) include a memoradum, T12, and rent roll. These, along with basic tax and local comp data, are used for preiminary underwriting to determine an offer amount.
Part of that underwriting is getting details from a lender (typically larger properties are leveraged with debt) on loan options. The lender will also do his own preliminary underwriting to determine estimated loan terms which would go into your underwriting.
After an offer is accepted, detailed underwriting goes along with due diligence (audits, etc.) to confirm any risk that may have been unknown during the preliminary.