The bid form sits in the front-end documents. It lists every number the owner wants and the order to give them. A bid with a blank or changed form can be rejected.
Subcontractors rarely fill in the owner's bid form. They still need to read it. It shows the alternates and unit prices the general contractor must collect from each trade.
Read the bid form before you start the takeoff. List each alternate and unit price that touches your trade. Structure your proposal so the general contractor can copy your numbers straight onto the form.