The item number (also known as article number) is a 5-digit number which is often mentioned on the product, for example on the bottom or the back. It can also be found on the back of the packaging box, usually in the bottom right-hand corner. In some cases, it may be followed by a hyphen and a version number.
The example image below shows 23386-11. The item number, in that case, is 23386. The number 11 behind the hyphen indicates the version.
Example: packaging box
Example: bottom of the product
Example: back of the product
Example: in a document