The use of the lookup path means that filenames used in the event type definition can be specified using relative paths rather than absolute paths. Consider for example the following import statement that is generated automatically by the Event Type Editor in each event type definition:

Here, the Event Type Editor correctly treats Event/Envelope.xsd as a relative path within the local copy of the predefined types.

If you open an event type in a schema editor other than the Event Type Editor, the editor will be unable to resolve this path, since it is unaware of the event type lookup path. This can happen, for example, if you validate a project (by clicking Validate in the project's context menu in the Project Explorer view); in this case, Eclipse uses its own schema checking to compile the event type and will generate a warning that the relative path cannot be resolved. You can ignore all such validation messages.