This setting appears under VPS: Lister Settings. It is specific to ProcessWire's Lister and is an optional setting that applies to both Modal Thumbs view and Modal Lister view. Use it if you want to set a default selector to find selectable pages. It can be used alongside an Initial Selector.
If used, this would be the default selector string that appears in the Lister but may be removed or changed by the user. The string you specify here must be a valid ProcessWire selector. An example selector could be: template=travel, title%=Japan.