FieldtypeSelector

string — a ProcessWire selector string (e.g. "template=product, sort=-created, limit=10"). Empty value is an empty string ''.

// Get the stored selector and use it to find pages
$selector = $page->selector_field;
if($selector) {
    $results = $pages->find($selector);
    foreach($results as $result) {
        echo $result->title;
    }
}

// Set a selector string programmatically
$page->selector_field = 'template=product, sort=-created, limit=10';
$page->save('selector_field');

// Clear the value
$page->selector_field = '';
$page->save('selector_field');

Stores text and supports fulltext/text-matching operators, the same as FieldtypeText:

// Find pages whose selector field contains a specific term
$pages->find('selector_field*="template=product"');

// Find pages with a non-empty selector field
$pages->find('selector_field!=""');