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In this example the option has been selected. When you click on the Employees node in the tree, the list of associated employees appear under it like this:
By default in side-by-side display the separate instance list actually appears underneath. To make it side by side, use the right mouse menu and then select Position and then Side by Side:
This is the type of display layout you will see:
The advantage of this type of display is that different children can have quite different sets of additional columns to their parent, or to each other.
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This property is in the Business Object Relationships tab.