I found the description of STD_PROP_LIST a bit vague, so I put together a simple demonstration to show its behavior:
<?php
$a = new ArrayObject(array(), ArrayObject::STD_PROP_LIST);
$a['arr'] = 'array data';
$a->prop = 'prop data';
$b = new ArrayObject();
$b['arr'] = 'array data';
$b->prop = 'prop data';
// ArrayObject Object
// (
// [prop] => prop data
// )
print_r($a);
// ArrayObject Object
// (
// [arr] => array data
// )
print_r($b);
?>