Libraries are a concept introduced by Microsoft in Windows 7. A library is a virtual folder that displays the contents of other folders. Unlike a file collection, which can store references to individual files and folders, a library displays the entire contents of one or more folders. You can't add individual files to a library - all you can do is change which folders make up that library. You can work with the files in a library (copy them, rename them, delete them, etc.) just like you would with the real files.
By default four libraries are created by Windows - Documents, Music, Pictures and Videos. You can restore these default libraries at any time by right-clicking on the root Libraries folder and choosing the Restore default libraries command.
Libraries are referenced internally using a URL-style path format, with the lib:// prefix. For example, the path of the default pictures library is lib://Pictures/. You can type this sort of path into the location field, or use it in buttons and hotkeys with the internal command set to automate your use of collections. For example, you could set up a button or hotkey to automatically navigate to the Pictures library using the raw command Go lib://Pictures.
The lib:// path system isn't recognised outside of Opus, so if you ever want to copy a library path to the clipboard and paste it into another program, you should use the Edit / Copy Other / Folder Path menu command rather than copying the contents of the location field.
The root Libraries folder holds all your libraries. To access the libraries root you can:
From the libraries root you can use the New Folder function on the toolbar to create a new library.
Newly created libraries initially have no member folders, and so will appear empty - you need to use the Properties dialog to include one or more folders in the library in order to use them. Right-click on your newly created library and select the Properties command to display it.
The list of member folders is displayed in the Library locations list in the Properties dialog.
Click the Include a folder button to add a new member folder to the library. When you click Apply or OK the contents of the new member folder will instantly appear inside the library. To remove a member folder, select it in the list and click the Remove button.
You can also add a folder to a library from the folder's right-click context menu (using the Include in library sub-menu).
Each library has a default location that new files are saved. If the library only has one member folder, that will always be the save location. If the library has more than one, the default save location is indicated with a checkmark in the Properties dialog. To change which member is the default save location, select it in the list and click the Set save location button.
When you navigate to a library, the file display shows the unified contents of all member folders. If your various member folders contain sub-folders or files with the same filename, it may not be obvious when looking at the library which item is which.
The Location column can help with this. It displays the true location of each item in the library, making it much easier to tell where they come from. You can add the Location column (and other columns) using the Folder Format dialog - or use the Folder Formats system to set the default format for a library.
If the Show member folders individually option on the Folder Tree / Contents page in Preferences is turned on, the folder tree will display an additional branch level for each member folder. This lets you use drag and drop to save new files into a specific member folder rather than the default save location.
You can hide an individual library from the tree by right-clicking it and selecting the Don't show in navigation pane option. To add it back to the tree, use the similarly-named option on the library's Properties dialog.