Toolbar buttons, menu items and hotkeys are constructed from one or more commands. There are two main types of commands, or instructions: internal and external.
Internal commands are the set of functions built-in to Opus - its "native language" if you like. There are around 30 commands that make up the internal command set, and each command takes one or more
arguments that defines or modifies its behavior.
External commands are other programs outside of Opus. Using one or more special
control codes, a toolbar button or hotkey can launch an external program and pass information like the names of selected files to it.
The third type of instruction that can be used in a button or hotkey is the command modifier. These are not commands themselves - instead, they modify the behaviour of the overall button.
More:
Argument Types
Internal Commands
External control codes
Command modifier reference