Your software license keys, that is. If you ever have to do a “clean” reinstall of OS X, that is, completely erase your hard drive and install a new copy of OS X, you will probably be presented with windows like the one shown below when you try to run some of your software after restoring your data from a backup.
I saw little windows like this about a dozen times after I had to do a clean reinstall of OS 10.9 Mavericks on one of my Macs because of problems after the normal “in-place” upgrade. The necessity for a clean install doesn’t occur very often, but I’ve been down this road a few times in my 20+ years with Apple computers and have learned how important it is to keep a record of software license information.
The reason you get the request to re-register your software after a clean install is because the registration data for the software has been lost from the System in the process. It can also happen if you have to uninstall and reinstall an application. Fortunately, this doesn’t happen with a restore of both the system and data from a backup, or a normal in-place upgrade.
In addition to the license key, you probably will also need the name in which the software is registered, usually your short or long user account name. The information isn’t needed for Software purchased from Apple’s App Store. It will run just fine, but software purchased elsewhere probably will require re-registration.
You can keep the license key information just about anywhere, but a really good place to keep it is in 1Password. In addition to all its other useful capabilities, 1Password has a section specifically for software you have purchased. Here is an example.
If you have lost license information for a particular app, you can contact the developer and sometimes they will provide it if you have registered the software with them.