Say you have a travel guide describing dogs-friendly accommodations or a special event and you want to list all accommodation that are relevant according to this article: You can run a search for all products linked to a specific guide using the <groupitemid> filter in a search component:

<component id="contextscasea" class="showcase">
<catalogue>accommodation</catalogue>
<dtcheckin>%-global.dtcheckin-%</dtcheckin>
<dtcheckout>%-global.dtcheckout-%</dtcheckout>
<groupitemid>%-form.cdo-%</groupitemid>
<usetemplate>%-system.lngtemplatepath-%hlib/comm/showcase3.html</usetemplate>
<renderto>component_contextscasea</renderto>
<retrieve>bizcard</retrieve>
<datasetalias>
<alias id="results">components.contextscasea.results</alias>
</datasetalias>
<sortby>name</sortby>
</component>

The value to be assigned to "groupitemid" is the guide's repuri.

Of course you can use this method to get any product starting from any of its "relatives", just mind to appropriately set the "catalogue" filter and make sure through the administration area that the repuri you are testing actually have linked products: for example, if you want to list all car rentals related to a hotel you will set the hotel's repuri in "groupitemid" and the "rental" value in "catalogue" filter (meaning you want to search all rentals related to the given repuri)