Inspecting NullAudio.c file from Apple's NullAudio example (see here) I found the following description:
// The purpose of the NullAudio sample is to provide a bare bones implementations to
// illustrate the minimal set of things a driver has to do. The sample driver has the following
// qualities:
// - …
// - a box
// - …
As stated in the title: What is meant by a box?