Besides the rather permanent classification into categories, individual files can be additionally marked. Three color markers are available for this purpose. They can be set via the context menu, the key combination Ctrl+Shift+1 (or +2, +3, to remove +u), or in the media list by clicking on the marker icon in the "Marker" column.
To view all marked files, click on the button next to the quicksearch. Then all files are displayed with the respective marker. Repeated clicking will switch the displayed marker accordingly.
In the display settings you can set whether files marked accordingly from the entire media library or only those from the current preselection are displayed.
The coverflow, search, quicksearch and other ways to search and browse the library will ignore these markers. The markers are stored only in the Media Library and not in the ID3-Tag of the music files.