nd field of the complementary reference field pair shall contain nemory_management_control_operation command equal to 6 that assigns the same long-term frame index to the second field. NOTE 5 - The above requirement must be fulfilled even when the first field of the complementary reference field pair is subsequently marked as "unused for reference" (for example, when a memory_management_control_operation, equal to 2 present in the slice header of the second field that causes the first field to be marked as "unused for reference"). When the second field (in decoding order) of a complementary reference field pair includes a memory_management_control_operation command equal to 6 that assigns a long-term frame index to this field and the first field of the complementary reference field pair is marked as "used for short-term reference" when the memory_management_control_operation command equal to 6 is processed by the decoding process, the decoded reference picture marking syntax structure of that second field shall contain either a memory_management_control_operation command equal to 1 that marks the first field of the complementary field pair as "unused for reference" or a memory_management_control_operation, command equal to 3 that marks the first field of Rec. ITU-T H.264 (01/2012) the complementary field pair as "used for long-term reference" and assigns the same long-term frame index to the first field. NOTE 6 - The above constraints specify that when both fields of a frame or a complementary field pair are marked as "used for reference" after processing all memory_management_control_operation commands of the decoded reference picture marking syntax structure, either both fields must be marked as "used for short-term reference" or both fields must be marked as "used for long-term reference". When both fields are marked as "used for long-term reference", the same long-term frame index must be assigned to both fields. difference_of_pic_nums_minus1 is used (with memory_management_control_operation equal to 3 or 1) 