From what I've observed, it's your ID, the database uses its own numbering. If you have 1000 books, each has it's own ISBN, but YOU could number them as well. If you don't do that, just number them sequentially and forget its even there.
![](https://ordasoft.com/components/com_fireboard/template/default_ex/images/english/emoticons/smile.png)
And yes, in the backend you can sort by that ID.
Shane