Obvious will the heads off!
The closed block has cast aluminium around the head gasket face with 'small' holes cast to allow the water passages to the heads.
The open block had almost the entire top od the block where the gaskets go open to water passage, hence the two phrases.
The CDB is the strongest due to the extra rigidity but only for high output engines.
My Sti v3 is open deck block and can run 407 bhp @ 1.41 bar.(all day long)
Graham.