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.