This setting appears under VPS: Lister Settings. As its settings header suggests, it is specific to ProcessWire's Lister. It is an optional setting that applies to both Modal Thumbs view and Modal Lister view. Use it if you want to set an initial selector to find selectable pages for your page field. The string you specify here must be a valid ProcessWire selector.
Please note that users will not be able to override the initial selector (except for limit). An example selector could be: parent.title!=album, limit=20, title^=b. Please also note that this setting does not override the ProcessWire settings for the page field, for instance, Parents of selectable pages or Custom selector to find selectable pages.