2.2.3 H.263 Payload Header, Draft Mode
The fields defined in the H.263 payload header in draft mode differ from the payload header in RFC mode in the following ways:
The orders of the following fields are rearranged: I, A, S, R, HMV1, VMV1, HMV2, and VMV2.
The sizes of the following fields are different: MBA, HMV1, VMV1, HMV2, VMV2, and R.
The H.263 payload header in draft mode does not specify a U field.
Details of these differences are specified in the following sections.