Minecraft wiki entry for Development_versions, associated text: Can now return multiple values.
When used as target, modification will be applied to every element.
When setting location and no elements are found, new matching element will be created:
Add [{k1:v1,k2:v2}] to match objects in list that have matching fields.
Add {k1:v1,k2:v2} to match objects.
Selects 0 or 1 elements, mostly as safeguard against mismatched entries.
Allows negative indices as [index] to index elements in reverse from end.
Add [] to select all elements from list.