Sharing an important concept of DAG in Exchange 2010 & 2013. This concept is there to avoid Split Brain from occurring. By default DAC mode is disabled it needs to be manually enabled.
When DAG is stretched between two data-centers & one datacenter hosts quorom , split brain can occur. If power outage occur & you activated the standby Datacenter, when the power is restored on the Primary data-center, it will have majority votes so it will try to mount the databases. This can led to issues.
DAC was brought to address this issue: Active Manager stores a bit in memory (either 0 or 1) that tells the DAG whether it’s allowed to mount local databases that are assigned as active on the server.
When a DAG is running in DAC mode, each time Active Manager starts up the bit is set to 0, meaning it isn’t allowed to mount databases. Because it’s in DAC mode, the server must try to communicate with all other members of the DAG that it knows to get another DAG member to give it an answer as to whether it can mount local databases that are assigned as active to it.
The answer comes in the form of the bit setting for other Active Managers in the DAG. If another server responds that its bit is set to 1, it means servers are allowed to mount databases, so the server starting up sets its bit to 1 and mounts its databases.