Based on some feedback from our customers, there has been some confusion regarding the role and function of the "alternate file share witness." The confusion lies in the event of datacenter activation; that the alternate file share witness would automatically come online as a means to provide quorum to the surviving DAG members and keep the databases mounted. So in many ways, some people view it as redundancy to the file share witness for an even numbered DAG.
In reality, the alternate file share witness is only invoked when an admin goes through procedures of activating the mailbox servers who lost quorum. DAC mode dramatically simplifies the process and when the “Restore-DatabaseAvailabilityGroup” cmdlet is executed during a datacenter activation, the alternate file share witness will be activated.
If the alternate witness server wasn't previously configured, you can configure it by using the AlternateWitnessServer and AlternateWitnessDirectory parameters of the Restore-DatabaseAvailabilityGroup cmdlet. For more information on this and the process of failing over DAG groups leveraging DAC mode, please refer to the Technet blog summary below.
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