When talking about classes, and how they relate, there are two classic relationships. Class Y can inherit from class X (inheritance), or class Y can use class X (composition). In the first case we say that an X is-a Y, in the second that an X has-a Y.

Sometimes it isn’t clear which choice is best, and you can imagine using either. One thought that can help decide: is it part of X’s essence that it is a Y? Is it impossible to think of an X that isn’t a Y? Or is Y just a helpful implementation detail of X, and there are other possible ways to build X?

I don’t want to open a can of worms debating licenses, and probably this is an absurd analogy, but it occurred to me that copyleft license (GPL) advocates view their license choice as an is-a, and permissive license (BSD, MIT, Apache) advocates view theirs as a has-a.