You can look at the catalog number as your item number on the release.
You make the number yourself and can consist of both letters and numbers.
For example, you can use your initials or on your record label, if you have one.
To keep track of your releases, you may want to create your own numbering system. For example:
Release 1: "My Record Mark" = MP 001
Release 2: "My Record Tag" = MP 002
And so on...
On CDs and LPs you will often find the catalog number printed on the back of the release.