ch coded macroblock, specifying the slice group to which each coded macroblock belongs. map unit to slice group map: A means of mapping slice group map units of a picture into slice groups. The map unit to slice group map consists of a list of numbers, one for each slice group map unit, specifying the slice group to which each coded slice group map unit belongs. Rec. ITU-T H.264 (01/2012) matrix: A two-dimensional array of syntax elements or variables. may: A term used to refer to behaviour that is allowed, but not necessarily required. In some places where the optional nature of the described behaviour is intended to be emphasized, the phrase "may or may not" is used to provide emphasis. memory management control operation: Seven operations that control reference picture marking. motion vector: A two-dimensional vector used for inter prediction that provides an offset from the coordinates in the decoded picture to the coordinates in a reference picture. must: A term used in expressing an observation about a requirement or an implication of a requirement that is specified elsewhere in this Recommendation International Standard. This term is used exclusively in an informative context. NAL unit: A syntax structure containing an indication of the type of data to follow and bytes containing that data in the form of an RBSP interspersed as necessary with emulation prevention bytes. NAL unit stream: A sequence of NAL units. non-paired field: A collective term for a non-paired reference field or a non-paired non-reference field. non-paired non-reference field: A decoded non-reference field that is not part of a complementary non-reference field pair. non-paired reference field: A decoded reference field that is not part of a complementary reference field pair. non-reference field: A field coded with equal to 0. non-reference frame: A frame coded with nal ref idc equal to 0. non-reference picture: A picture coded with nal ref ide equal to 0. A non-reference picture is not used for inter prediction of any other pictures. 