I notice there's a check for message_start in the function ReadRawBlockFromDisk.

https://github.com/bitcoin/bitcoin/blob/master/src/validation.cpp#L1207

If I understand correctly, message_start is used to identify Bitcoin Core clients.

https://bitcointalk.org/index.php?topic=2997687.0

Why does the function ReadRawBlockFromDisk check for message_start? Does that mean message_start is also written into blocks?

Thanks.