Bills of Materials in Cofactr are the list of electrical components that you are sourcing from Cofactr. They are often created, at least initially, in KiCad, Altium, or other ECAD software. Usually, any BOM that would be sent to a Contract Manufacturer will work great with Cofactr.
The only data Cofactr needs to source your parts is the MPN (Manufacturers Part Number) and either the quantity or the reference designators (which would tell us the quantity inherently).
BOMs are most frequently uploaded by electrical engineering teams so they can compare supply of components as they design products. Cofactr does a great job of helping design for supply chain during the New Product Introduction (NPI) phase of design or when a finished BOM needs to be sourced. On average, Cofactr will match more parts to better sources across a BOM than any other tool.