What is This?
 

In Synaptics Package Manager, I see sections like "installed" and whatnot & there's one called "Not Installed (residual config).

Do I need to install these?
Also, one of them says "libktoblzcheck1v5" which apparently checks for bank verification codes or something.

Is this normal?

When you install a package, there are normally three types of files installed/modified:

The application itself + system configuration files + user configuration files

When you remove a package, only the first of these is actually removed, i.e. the application itself. If you also want the system configuration files to be removed, you can purge the package rather than remove it (although you can also do this as a separate action after removal). I don't use Synaptic but I think that it calls the purging of a package its "complete removal".

In either case, the user configuration files are always left behind. These are normally files in your home directory, usually dot files (existing in a directory that starts with a dot). They are left so that you can reinstall a package and retain all your user settings for that package. They must be deleted manually as a separate action if you so desire.

Anyway, packages that are marked as having "residual configuration" are packages that you previously had installed on your system but which you removed, but didn't purge in order to remove the system configuration files.

In my experience I have never needed to remove a package and not go ahead and purge it. I recommend therefore that you completely remove those packages flagged as "Not Installed (residual config)" and that in the future you purge all packages when removing them.

Thanks for the reply. Do you know if that package I named is safe and why it would be on my pc?

It must have been previously installed on your system then removed. So, either it came with your default install or you installed it, either explicitly or as a dependency of another package.

The following command:

shows that the package in question is in the Ubuntu bionic repositories (on my system anyway, you have given no indication as to which distro and version you are running), while:

indicates that it is a:

"library for verification of account numbers and bank codes libktoblzcheck is a library for verification of bank account numbers and bank codes (BLZ) of German Banks. It is based on the specifications of the "Deutsche Bundesbank". It also supports the verification of international bank account numbers (IBAN) and bank identifier codes (BIC)."

Looks harmless enough to me.

The command:

shows that the following packages depend on it:

The most likely suspect among those is GnuCash. Did you ever have that package installed on your system?

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