From the "Tiny book of Rules":
Invoking components programmatically Rules components are handy tools for allowing site builders to customize their own actions and conditions. If you are writing a module that allows site builders to set custom conditions (or actions), you can let them set these up as Rules components and then evaluate selected components programmatically. This works in a way analogous to invoking events, by using rules _invoke_component() . The first argument is the (machine) name of the component, and any subsequent arguments are variables sent to the component. // Included somewhere in your custom module. $component name = $settings [' rules component name']; // Any variables provided from the component are returned from its evaluation. $result = rules _invoke_component ( $component_name, $var 1, $var 2 ) ;
So the last 2 lines in the quote above should answer your question.