11.4.9.4 Update a Collection Property: reference section 11.4.11 Positional Inserts

Add a reference and align the text. A successful POST request to the edit URL of a collection property adds an item to the collection. The body of the request MUST be a single item to be added to the collection. If the collection is ordered, the item is added to the end of the collection, and if the collection supports positional insert $index MAY be used to specify the insert position a zero-based ordinal position to insert the new value, with a negative value indicating an ordinal position from the end of the collection . Explicitly mention that the success response for return=representation is the after-image of the collection, same as for a GET request to the collection property. Upon successful completion the service responds with either  200 OK   and a representation of the updated collection, or  204 No Content . The client may request that the response SHOULD include a body by specifying a  Prefer header  with a value of  return=representation .