The <o:componentIdParam> component allows to render just one or more components on a view via a GET parameter.
Components can be identified via both their client id or simple component id. Via the former it's possible to e.g. render only a specific row in a table. For specific cases, it's possible to render only the parent component and omit any children.
Among the use cases for this is creating simple mashups from various JSF based views, and scripts needing to obtain markup for specific components on an initial (non-faces) request.
Note that this is a rather specialized component and for many common use cases the user is advised to investigate if the existing AJAX and partial page requests in JSF don't already cover the requirements. For the moment this component only supports the direct output of the original markup and does not wrap it into any "partial response" envelope.
Remember to pass start index as <f:param> along command links/buttons inside repeated item,
so that the model can properly be preinitialized to identify the repeated item on which the action is invoked.
See also the "select" command link source code of this example.