I would say its index/long list to content that deemed illegal.
It's going to be a hash table of SHA256 checksums, exactly which part of the transactions or blocks are going to be hashed I haven't figured it out yet.
You could start with,
1. Data on OP_RETURN output.
2. Data pushed with certain opcodes (e.g. OP_PUSHDATA).
3. Pubkey on P2MS output. You'll want to read SRC-20 specification.