The Commands page displays a list of commands that you can easily add to toolbars by drag and drop. The commands are divided into a number of categories that loosely groups similar commands (often commands are grouped based on where they appear in the default toolbars).
Opus functions are built from a set of internal commands (like a simple scripting language), and each command takes multiple parameters ("arguments") that control its behavior. While this can be very flexible it does require delving into the bowels of Opus somewhat, and so many common functions have been supplied in the list on this page as pre-defined commands.
The pre-defined commands have user-friendly names, and if you click on them a description of the command is displayed to the right of the page. To add one of the commands from this list to your toolbars, simply drag it from the list and drop it where you want it to go. When dropped on a toolbar they resolve to their underlying command and arguments in the button that's created, so if you edit one of these buttons after dropping it on a toolbar you can see what the "real" command is.
At the bottom of the Customize dialog (when it's showing the Commands page) is a field that lets you filter the commands list.
When you enter one or more keywords in the field and press the Enter key the list will be filtered to only show those commands whose name or description contains the keyword(s). This can make it very easy to find a pre-defined command when you remember its name but not what category it is in. To clear the filter either press Escape when the cursor is in the field, or click the little X button at the right-hand end of the field.
The category list on the Commands tab works the same as the similar list on the Preferences dialog. You can use the keyboard to navigate it: Cursor Up / Down to move through the list, Cursor Right / Left to expand or collapse a category, * to expand all categories and - twice to collapse all categories.
The Import and Export commands in the File menu for this page let you import and export user-defined commands.