This project is based on the excellent work of Tim Geyssens.
The datatype in this package works exactly like his well-known Ultimate Picker but surprisingly, also accepts XPath as parent nodeid.
$currentPage allows you to use the picker to select children of the node that is being edited.
You can also use $currentParent because it only works on published nodes. If the node is not published it's not in the xml and therefore can't be used.
$currentParent/ancestor-or-self::node [@level = 1] allows you to select the root of the website you are currently editing. This can be very useful when working with multilingual websites.