Many questions here on SO ask about custom classes. I, on the other hand, have no idea what they're talking about. "Custom class" seems to mean the same thing I mean when I say "class".

What did I miss, back in the '80s, that keeps me from understanding?

I know that it's possible to purchase a packaged system - for Accounting, ERP, or something like that. You can then customize it, or add "custom code" to make the package do things that are specific to your business.

But that doesn't describe the process used in writing a .NET program. In this case, the entire purpose of the .NET Framework is to allow us to write our own code. There is nothing useful out of the box.