Typically, serial numbers are issued by production control at the end of the manufacturing line. Manufacturing must record the shipment date of each serial number for warranty reasons. The serial number is typically a three- or four-digit identifier based on the day the product was shipped out and the year it was manufactured. Warranty coverage can only be established if both the manufacturer and the original retail seller know what date was put on box lid or packaging. If these two events are more than sixty days apart, the warranty will not apply. There are two types of serial numbers: made-up numbers and numeric codes. Made-up numbers are used when there is no standard procedure for assigning serial numbers. For example, they may be used when products are hand-finished to identify each unit sold. Numeric codes are used when there is a standard process for assigning serial numbers. These become necessary when many products need to be serially numbered, such as with product revisions or changes to manufacturing processes. A numerical code allows for repeatable numbering across a large set of products. The first step in assigning a serial number is to determine whether it is a made-up number or a numeric code. This can be done by looking at how the manufacturer intends to use the number.