The Preload Reference Editor is used when you want to select specific file references to be loaded or unloaded when the scene is opened. The Preload Reference Editor appears when you open a file when the Selective Preload option is turned on.

To selectively load file and proxy references using the Preload Reference Editor
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            The file browser appears, listing the default scene directory for your project.
The Preload Reference Editor appears. The Preload Reference Editor lists all available file references for the scene. Depending on how the Referencing options were set, some file references will appear loaded or unloaded in the Preload Reference Editor. When an icon appears beside a particular file reference it indicates that one or more proxy references exist for that reference.
 -click
                     an item and select Bring in Reference Loaded or Bring
                        in Reference Unloaded from the menu that appears.
 -click
                     an item and select Bring in Reference Loaded or Bring
                        in Reference Unloaded from the menu that appears.
                   -click
                     an item and select Set Active Proxy To from the
                     menu that appears. When a file reference contains one or more proxy
                     references, the Set Active Proxy To option
                     appears. Set Active Proxy lets you select
                     which proxy reference you want to load into the scene by displaying
                     the list of available proxy references (by their proxy tags) in
                     the Set Active Proxy To submenu.
 -click
                     an item and select Set Active Proxy To from the
                     menu that appears. When a file reference contains one or more proxy
                     references, the Set Active Proxy To option
                     appears. Set Active Proxy lets you select
                     which proxy reference you want to load into the scene by displaying
                     the list of available proxy references (by their proxy tags) in
                     the Set Active Proxy To submenu.