Root interface allowing a logical unit (variable, library) to contain more than one interface.
The container is a required element to define the base of a logical unit. It contains basic information on the build being used by the enclosed plug-ins. This allows the host to quickly determine if it should pursue loading this container or not, and might need to apply patches to fix interface variations appearing between SDK versions.
The container being versioned means this format could eventually change, but the goal was to make it generic enough to keep it stable. It's easier to add up plug-ins to extend the format than to modify it.