RLC PDU is a bit string. In the figures in clause 18.104.22.168
to 22.214.171.124, bit strings are represented by tables in which the first and most significant bit is the left most bit of the first line of the table, the last and least significant bit is the rightmost bit of the last line of the table, and more generally the bit string is to be read from left to right and then in the reading order of the lines.
RLC SDUs are bit strings that are byte aligned (i.e. multiple of 8 bits) in length. An RLC SDU is included into an RLC PDU from first bit onward.
TMD PDU consists only of a Data field and does not consist of any RLC headers.
UMD PDU consists of a Data field and an UMD PDU header. The UMD PDU header is byte aligned.
When an UMD PDU contains a complete RLC SDU, the UMD PDU header only contains the SI and R fields.
An UM RLC entity is configured by RRC to use either a 6 bit SN or a 12 bit SN. For groupcast and broadcast of NR sidelink communication or for SL-SRB4, only 6 bit SN length is configured. An UMD PDU header contains the SN field only when the corresponding RLC SDU is segmented. An UMD PDU carrying the first segment of an RLC SDU does not carry the SO field in its header. The length of the SO field is 16 bits.
AMD PDU consists of a Data field and an AMD PDU header. The AMD PDU header is byte aligned.
An AM RLC entity is configured by RRC to use either a 12 bit SN or a 18 bit SN. The length of the AMD PDU header is two and three bytes respectively.
An AMD PDU header contains a D/C, a P, a SI, and a SN. An AMD PDU header contains the SO field only when the Data field consists of an RLC SDU segment which is not the first segment, in which case a 16 bit SO is present.
STATUS PDU consists of a STATUS PDU payload and an RLC control PDU header.
RLC control PDU header consists of a D/C and a CPT field.
The STATUS PDU payload starts from the first bit following the RLC control PDU header, and it consists of one ACK_SN and one E1, zero or more sets of a NACK_SN, an E1, an E2 and an E3, and possibly a pair of a SOstart and a SOend or a NACK range field for each NACK_SN.