How can you assign a part number internally, if you don't know what it is? Generally, in the design phase, an internal part number may or may not be assigned until the product goes to production. This is usually accompanied by a list of acceptable parts (listing mfg and part number). Listing more than one mfg is usually a good idea to have an alternate source. Also, being the designer, you know what is important in the part your selecting so listing alternates is a good idea to prevent unacceptable parts being used later by another engineer less intimate with the project.