Each block:

HEAD_CRC       2 bytes     CRC of total block or block part
HEAD_TYPE      1 byte      Block type
HEAD_FLAGS     2 bytes     Block flags
HEAD_SIZE      2 bytes     Block size
ADD_SIZE       4 bytes     Optional field – added block size

Field ADD_SIZE present only if (HEAD_FLAGS & 0×8000) != 0

Blocksize:

if (HEAD_FLAGS & 0×8000) == 0  && HEAD_SIZE+ADD_SIZE 
	if the field ADD_SIZE is present – when
(HEAD_FLAGS & 0×8000) != 0.

Types of blocks

HEAD_TYPE=0×72          marker block # first block
	The marker block is actually considered as a fixed byte
	sequence: 0×52 0×61 0×72 0×21 0x1a 0×07 0×00
HEAD_TYPE=0×73          archive header
	
HEAD_TYPE=0×74          file header
HEAD_TYPE=0×75          old style comment header
HEAD_TYPE=0×76          old style authenticity information
HEAD_TYPE=0×77          old style subblock
HEAD_TYPE=0×78          old style recovery record
HEAD_TYPE=0×79          old style authenticity information
HEAD_TYPE=0x7a          subblock



HEAD_FLAGS:
0×0001  – Volume attribute (archive volume)
0×0002  – Has comment (v3 uses the separate comment block and does not use this)
0×0004  – Archive lock attribute
0×0008  – Solid attribute (solid archive)
0×0010  – New volume naming scheme (‘volname.partN.rar’)
0×0020  – Authenticity information present RAR 3.x does not set this flag.
0×0040  – Recovery record present
0×0080  – Block headers are encrypted
0×0100  – First volume (set only by RAR 3.0 and later)
0×4000 – if set, older RAR versions will ignore the block
0x8000 - if set, ADD_SIZE is present and bs = HEAD_SIZE + ADD_SIZE