chan.arc is an in-development standard for storing image board threads.
While WARC should be used for all new image board archiving, there are still lots of old threads that are only available as Fuuka database dumps, and archives that read thread information into a proprietary database format. When you can't install the software anymore, or the files become unimportable, those threads would be lost.
chan.arc is an attempt to standardize a file format that can be used to store these sort of image board threads. It can also be used by home archivers to ensure that Fuuka-like archives can import and use their work.
The draft specification is available to read on Github.
PLEASE NOTE: [This specification is still in development ]