Opus supports various command modifiers that can be used in toolbar buttons and hotkeys to modify the behaviour of the function. A command modifier is not a command itself - for the most part you can think of them as options that control how the function is executed.
Each modifier is given on a separate line - therefore, to use command modifiers in a function, you must edit the function in the advanced command editor (as the simple editor does not support multiple-line functions). Some modifiers (like @async) affect an individual command in the function, and are given on the same line as the command - others affect the whole function, and are supplied on a line by themselves.
This screenshot is an example of a simple function that makes use of two command modifiers:
See the Command modifier reference page for a full list of the available command modifiers.