That is the biggest box in the VRML world. This system is signified by the first separator node right after the VRML declaration. All VRML 1.0 worlds have a root separator. Any new separators should be inside this one. Call the first separator, the root separator.
Any separator can include separators inside it, or at the same level.
This is sometimes referred to as a parenthetical nodelist. You can think of them as boxes inside boxes. Level1A. Level1B is inside root, and so on. In the next section on transforms, you will see why this nesting of separators is needed. Some VRML 1.0 browsers allow you to get away without having a root separator.