003. When the three-byte sequence 0x000001 is detected, this is a start code prefix. When the three-byte sequence 0x000003 is detected, the third byte (0x03) is an emulation_prevention_three_byte to discarded as specified in clause 7.4.1. When an error in the bitstream syntax is detected (e.g., a non-zero value of the forbiddenz or one of the three-byte or four-byte sequences that are prohibited in clause 7.4.1), the decoder may consider the detected condition as an indication that byte alignment may have been lost and may discard all bitstream data until the detection of byte alignment at a later position in the bitstream as described in this clause. Rec. ITU-T H.264 (01/2012) Annex C Hypothetical reference decoder (This annex forms an integral part of this Recommendation | International Standard.) This annex specifies the hypothetical reference decoder (HRD) and its use to check bitstream and decoder conformance. Two types of bitstreams are subject to HRD conformance checking for this Recommendation | International Standard. The first such type of bitstream, called Type I bitstream, is a NAL unit stream containing only the VCL NAL units and filler data NAL units for all access units in the bitstream. The second type of bitstream, called a Type II bitstream, contains, in addition to the VCL NAL units and filler data NAL units for all access units in the bitstream, at least one of the following: additional non-VCL NAL units other than filler data NAL units, all leading_zero_8bits, zero_byte, start_code_prefix_one_3bytes, and trailing_zero_8bits syntax elements that form a byte stream from the NAL unit stream (as specified in Annex B). Figure C-1 shows the types of bitstream conformance points checked by the HRD. Non-VCL NAL units other VCL NAL units than filter data NAL units Filter data NAL units Byte stream format encapsulation (see Annex B) Type II HRD Type II HRD Type I HRD conformance point when conformance point when conformance point not using using byte stream format byte stream format H.264(09)_FC-1 Figu