Activity Streams 2.0
https://www.w3.org/TR/activitystreams-core/#collections
> The items within a `Collection` can be ordered or unordered. The `OrderedCollection` type MAY be used to identify a Collection whose items are always ordered. In the JSON serialization, the unordered items of a Collection are represented using the `items` property while ordered items are represented using the `orderedItems` property.
それは Vocabulary でなく Streams の方で言及されている (なんじゃこのガバ規格は)