le 7-18, where the row for "inferred" specifies values inferred when sub_mb_type[ mbPartIdx ] is not present, and the inferred value "mb_type" specifies that the name of sub_mb_type[ mbPartIdx is the same as the name of mb_type for this case. Table 7-18 - Sub-macroblock types in B macroblocks inferred mb_type Direct B_Direct_8x8 Direct B_L0_8x8 Pred_L0 B_L1_8x8 Pred_L1 B Bi 8x8 BiPred B_L0_8x4 Pred_L0 B_L0_4x8 Pred_L0 B_L1_8x4 Pred_L1 B_L1_4x8 Pred_L1 B_Bi_8x4 BiPred B_Bi_4x8 BiPred B_L0_4x4 Pred_L0 B_L1_4x4 Pred_L1 B_Bi_4x4 BiPred The following semantics are assigned to the sub-macroblock types in Table 7-18: B _Skip and B Direct 16x16 no motion vector differences or reference indices are present for the sub-macroblock in the bitstream. The functions SubMbPartWidth() and SubMbPartHeight( ) are used in the derivation process for motion vectors and reference frame indices in clause 8.4.1 for direct mode prediction. B _Direct_8x8 no motion vector differences or reference indices are present for the sub-macroblock in the bitstream. The functions SubMbPartWidth( B_Direct_8x8) and SubMbPartHeight(B_Direct_8x8) are used in the derivation process for motion vectors and reference frame indices in clause 8.4.1 for direct mode prediction. X_MxN, with X being replaced by L0, L1, or Bi, and MxN being replaced by 8x8, 8x4, 4x8 or 4x4: the samples of the sub-macroblock are predicted using one luma partition of size MxN equal to 8x8, or the samples of the sub- macroblock are predicted using two luma partitions of size MxN equal to 8x4, or the samples of the sub-macroblock are predicted using two luma partitions of size MxN equal to 4x8, or the samples of the sub-macroblock are predicted using four luma partitions of size MxN equal to 4x4, and associated chroma samples, respectively. All sub-macroblock partitions share the same reference index. For an MxN sub-macroblock partition in a sub- macroblock with sub_mb_type[ mbPartIdx ] being B X MxN with X being replaced by either L0 or L1, one motion vector difference is present in t