What is it for?
A Serial Number is a code composed of a number of letters or numbers, given to a product or series of products, to identify the place and time of manufacture, determine the legality of its origin, and sometimes also the features of the device, such as color or storage capacity.
For many mobile phones based on serial numbers we can check out a lot of information about the phone. You can often check the date of manufacture, country of origin or warranty period.
The serial number does not have a structure like the IMEI number. Each manufacturer assigns it according to its scheme and defines it with its various features and characteristics.
Where to find it?
The serial number is usually located on the phone box, under the battery and can be found in the phone menu. Sometimes the serial number is also saved on the bill of sale or on the carrier's website after logging in to your account.
Serial numbering is a kind of theft protection, the device with the number can be register and in the case of theft are easy to identify.
This is also important for quality control when a defect is detected in the production of a specific product batch. Based on SN, the manufacturer can easily check the factory, the production line and the exact time when the defective product was released.
Serial Number in Apple Devices
CHECK IMEI 
