The Directory Opus folder alias system lets you assign simple names (aliases) to folders. Say you had a long, complicated path to a folder that you use frequently - for example, C:\Users\Jon\Documents\Company\Spreadsheets\Sales Projects\2011. Using the alias system you could assign a name like Sales11 to this folder. You can then use this alias anywhere in Opus that you would ordinarily use the full path. For example, you could click in the location field and enter /Sales11 to navigate to that folder (all aliases are prefixed with a forward-slash when they are used).
There are also a large number of built-in aliases that are predefined to the location of common system folders. For example, /dopusdata is a built-in alias for the folder that stores your personal Directory Opus configuration files. The location of this folder changes depending on your user name and your operating system, but you don't need to know this or be aware of the details - you can simply use the alias to find it.
The alias system also allows you to refer to drives by volume label (or name) rather than drive letter. The format for this is /$<label>. For example, you may have a USB thumb drive labelled "My_Portable" that you want to refer to in Opus commands. One problem with removable drives is that they can be assigned different drive letters from one use to the next. Using the alias /$My_Portable would refer to this thumb drive no matter what drive letter had been assigned to it. Of course if you have two or more drives with the same label the behavior of this will be unpredictable - Opus simply uses the first drive with the specified label that it finds.
You can also use aliases in buttons that you define yourself (for example, a button with the command Copy TO /Sales11 would automatically copy any selected files to the aliased folder).
As well as making it easy to remember and use complicated folder paths, another advantage of aliases is that you can change the folder an alias points to without having to manually update any references to that alias. In the above example, instead of using /Sales11 as the alias, you could use something like /CurrentYrSales - then, at the start of the next financial year, simply change the folder that your alias points to to the 2012 directory and any buttons or references to it will automatically use the new location.
You can define your own aliases or see a list of the built-in ones from the Favorites and Recent / Folder Aliases page in Preferences.