Simple definitions of these items are found below. It is important to understand that not all products will have a UPC or Manufacturer ID. Much of that depends on whether the manufacturer has gone to the effort of creating these.
UPC - The Universal Product Code (UPC) is an assigned number the manufacturer can obtain for a product through a registration process. This code is a completely unique identifier for that product. Many product will not have a UPC due to the amount of effort required. The UPC contains only numbers.
Manufacturer ID - This is typically the identifier assigned to a product by the manufacturer. These codes can contain letters, numbers, and simple punctuation such as hyphens, colons/semicolons. These codes should not contain spaces.
SKU - This is an identifier specifically for your unique use. You can create your own SKU codes according your preference, you can also click in the SKU field to have the system create a unique SKU number for you, as pictured below. These codes can contain letters, numbers, and simple punctuation such as hyphens, colons/semicolons. These codes should not contain spaces.