https://github.com/Bulat-Ziganshin/FA/blob/master/FreeArc-archive-format.md
https://encode.ru/threads/2621-FreeArc-Next?highlight=arc
https://encode.ru/attachment.php?attachmentid=4699&d=1475957410
An archive consists of blocks. This includes so-called SOLID BLOCKS, containing contents of files stored in the archive, and all remaining block type, collectively called CONTROL BLOCKS, holding meta-information about archive and files it contain.

Currently, there are the following types of control blocks:

HEADER block is the first block of any archive. It starts with FreeArc arhive signature, plus contains info about archiver version.
DIRECTORY blocks describes solid blocks stored in the archive, and files whose contents stored in these solid blocks
FOOTER block is the last block of the archive. It describes DIRECTORY blocks stored in the archive plus contains common archive information such as archive comment.
RECOVERY block, if present, is placed after all archive blocks including the FOOTER block. It contains ECC data that may help to restore damaged archive.
Each directory block is placed right after the solid blocks it describes. Archive may contain multiple directory blocks and multiple solid blocks per directory block.

Numbers in control block are stored in variable 1-9 byte format, except for CRC/time/signature having a fixed width of 4 bytes. Block type and boolean flags are stored as 1 byte. Strings (filenames, compression/encryption algorithms) are stored with trailing NUL byte. Lists are preceded with number of their elements and stored in the struct-of-arrays order (as opposite to array-of-structs). CRC algorithm used is pkzip's CRC-32.