This window appears when you click the Pre-join button 
on the Catalog Browser toolbar, select a data source and then click OK. It helps you to edit pre-joins in the selected data source of the current catalog, which can be used for creating/editing queries and setting up joins in business cubes. See the window.
You can make joins between tables by using mouse drag and drop. Point to one column in the source table, press and hold the mouse button, then move the pointer to the other column in the target table and release the mouse button. A green line with a join button 
will be shown, linking the two columns.
If you want to further edit a join, click its join button and the Join Options dialog will be displayed for you to edit it.
After you have made some joins between the tables, you can then use them to define paths. To define paths, click the Paths button on the toolbar of the editor.
OK
Applies all the changes and closes this dialog.
Cancel
Does not retain any changes and closes this dialog.
Paths
Opens the Save Pre-join Path dialog to define pre-join paths.
Arrange
Organizes the tables in the table panel.
Help
Displays the help document about this feature.
