A selection is a container of resources, returned as a response to a request. For example, if you ask for all the properties of a specific resource, a selection will be created containing all the statings that has the resource as subject.
An example of piling selections:
$interface->select( type=>$Person )->select(
memberOf=>$Group )->select( livesIn=>$Country )->name;
Would be the same as:
$service->select( interface=>$interface, type=>$Person,
memberOf=>$Group, livesIn=>$Country )->name;
There are many little things that can be done to efficiently determine the list of resources contained in a selection. There will be the same types of considerations as in the optimization of a database SQL query.
There are selections. But that API is not implemented.