$_data : array
Component specific global data storage area, which should be used for stuff like default configurations etc. thus avoiding the pollution of the global namespace. Each component has its own array in the global one, allowing storage of arbitrary data indexed by arbitrary keys in there. The component-specific arrays are indexed by their name.
Note, that this facility is quite a different thing to the component context from midcom_application, even if it has many similar applications. The component context is only available and valid for components, which are actually handling a request. This data storage area is static to the complete component and shared over all subrequests and therefore suitable to hold default configurations, -schemas and the like.