The $SiblingOrder is the top-to-bottom order, as seen in an outline, of siblings with a common container (including root level notes. The siblings number from one and the $SiblingOrder is a read-only calculated number. Within a set of siblings with no descendants, the sequence of $SiblingOrder will match the notes' $OutlineOrderSequence. However, if any note has descendants, all the latter list in $OutlineOrder before the next sibling note of their ancestor (see the previous article on $OutlineOrder for more).
In export contexts, the outline view is normally used which does not show adornments. Thus the last note in a container may not be the last outline order item in the container although it is the last exportable item. To help avoid the latter problem, the $SiblingOrder system attribute ignores adornments making it easier to detect unambiguously the first/last item in the container during export.
Whilst in Outline, $SiblingOrder is easily intuited
Next… Map co-ordinates