Every PUT-request by a sender contains an identifier for the object that it puts into the target application. Often this identifier-value has been retrieved from the target application earlier on, and the target application is the manager of the identifier-values. In that situation, it is not possible for the sender to create a new object by posting a PUT-request, because the sender can not determine the identifier-value of the new object.
However, if the sender is the manager of the identifier-value ("owns the identifier"), and not the target application, then the sender can create the identifier-value, pass it along in the PUT-request (along with any other information of the object-to-be-created), and make the target application create a new object in the target application.