Unused, Used and All filters
The library shows Unused by default so you don't place the same photo twice. Switch to All to see everything with used photos dimmed, or Used to see only what's placed.
Starts at 13:47 in the full tutorialWatch on YouTube
More in Photo library
See all 11Filter by star rating14:29Filter by color label14:45Preview a photo large (spacebar)15:52Find a photo by file name or number16:42Sort the photo library16:25Show file names and file numbers15:23Photo library view options58:44Make library thumbnails bigger or smaller1:33Folders in the photo library0:27Filter, group and search photos by folder0:41