CRBM and CRFM are 2 new feature’s in Exchange 2010 SP1.Exchange 2007/2010RTM relied on log shipping from an Active node to a Passive one which means, after a 1mb log file is committed onto the active database, it will get shipped to the passive copy.
With the release of SP1, Continuous Replication File Mode (CRFM) and Continuous Replication Block Mode (CRBM) are being introduced.
CRFM is the feature responsible for replicating the active database to the passive one by ensuring all logs are copied and upto date on the passive node. Once all logs are upto date, that’s when CRBM comes into picture. With CRBM or block mode, each update on an active database copy is also written onto the passive copy simultaneously. So in the event if the server holding the active database goes down, the passive copy will have all of the updated information to mount itself.
NOTE: CRBM will only be active if all logs are replicated to the passive copy and are upto date.