A proforma tells you the Gross Income Multiplier (GIM) and the CAP (capitalization) rate for the property. But they come in many different versions, so there is no real standard to consider. Hopefully you will also learn what current rent rates are overall along with income and expenses and other building details. Ideally it would include the unit mix, price per unit, rent per square foot, overall square footage, amount of acreage, assessed values and details about unit amenities too.