You can create extended QMF catalogs to organize user access to objects. Instead of all objects going into the main default catalog, they can go into smaller extended catalogs.
There are many reasons to use extended catalogs, for example, you can control users views of objects. You can isolate objects from one database server that is sharing a QMF catalog by creating an extended catalog. The extended catalog will contain only the selected objects from a specific database server. You can also specify that multiple database servers can share extended catalogs.
That means, an extended catalog can contain objects from several database servers. The ability to have multiple extended catalogs residing on different database servers can affect performance, the user's views of objects and overall maintenance of your query environment.