It's a term I made up. I looked around and didn't find a suitable term in the existing literature, and it fit my needs. I wanted to get across that it's all a matter of perspective. One person's production code, which is the code under test, is another person's containing code. For example. In my day job I help maintain an implementation of JSF. For me, that's my production code. However, to most users, that's containing code, because their code runs on top of JSF. This perspective goes all the way down the stack, through the seven layers of the TCP spec, to the bare metal, etc.