Extension Reference Types 
If you select Object Reference as the data type for an extension definition, you must create an extension reference type and link it to the extension definition.
This section describes how to create and edit an extension reference type.
To create an extension reference type:
Choose Setup in the toolbar at the top of the page.
Choose the System Setup tab.
In the Configuration section, select Extension Reference Types .
On the Extension Reference Type List page, choose Create.
On the Extension Type page, fill in the fields with extension reference type information.
Save your entries.
To edit an extension reference type:
Choose Setup in the toolbar at the top of the page.
Choose the System Setup tab.
In the Configuration section, select Extension Reference Types .
On the Extension Reference Type List page, do one of the following:
From the dropdown list, select Search Extension Reference Types by Name, search for the type to edit, and choose the type.
From the dropdown list, select All Extension Reference Types or All Inactive Extension Reference Types and choose the type to edit.
On the Extension Type page, choose the Edit in the toolbar.
Edit any fields.
Save your changes.