SSBs are part of the scalability feature that allows a PDomain to control more than one DCU. See Scalability Hardware for an overview.
Scalability boards (SSBs) are configured to work together as a single unit. If an SSB fails, the server attempts to remove the faulty SSB from the configuration in order for the scalability feature to continue functioning. If the reconfiguration succeeds, the server continues working.
Note - If the server contains expandable PDomains and an SSB is replaced, the PDomains must be stopped to allow the scalability unit to be reconfigured with the new SSB. If there are no expandable PDomains, the stoppage is not necessary. See Remove an SSB to determine if a PDomain is expandable.