typedef void * XPLMCommandRef;
A command ref is an opaque identifier for an X-Plane command. Command references stay the same for the life of your plugin but not between executions of X-Plane. Command refs are used to execute commands, create commands, and create callbacks for particular commands.
Note that a command is not "owned" by a particular plugin. Since many plugins may participate in a command's execution, the command does not go away if the plugin that created it is unloaded.