A frequently asked Question: "In the Browser several images are selected, but if you try to assign keywords, rating, or other metadata tags, they are applied to only one of the selected images - why?"
The reason for this usually is, that the "Primary Only" flag is set. With this flag enabled, changes to the keywords, ratings, and other metadata will be applied to only the "primary selection": The primary selection is the actively selected image; this image is marked by a thick white border in the Browser and the Viewer, see the paragraph "Navigating Through and Selecting Images" in the Aperture 3 User Manual.
To enable or disable the "Primary Only" flag, toggle the "Primary Only" button marked by a "1" in the Display Controls, or use the Aperture main menu bar:
Edit > Primary Only.
The borders of the images in the Viewer mark the status of the "Primary Only" switch: When off, all selected Images in the Viewer have a white border, with the Primary Selection showing a heavy border. When on, only the Primary Selection shows a border.
The screenshot below shows a selection of six images, with the primary selection highlighted by a thick, white border and the "Primary Only button" (marked by a red arrow) enabled. Notice that the six selected images are framed by thin, white borders in the Browser, but not in the Viewer.
Six selected images in the browser and the viewer, with "Primary only" disabled:
Thanks a lot to Kirby Krieger for proof-reading and very helpful suggestions!