After the XtremlO X1 has been racked and the cabling has been checked, you now...

 Power the PDUs

 Power on the BBUs

 Power on the Storage Controllers

 Power on the physical XMS Server

Power the PDUs: The Power Distribution Units (PDUs) need to be powered first as they supply power to all the other components.

Power on the BBUs: The Battery Backup Units (BBUs) are powered on next to ensure that the system has emergency power in case of a power failure.

Power on the Storage Controllers: The storage controllers are responsible for managing the storage array, so they need to be powered on before the server.

Power on the physical XMS Server: Finally, the XMS server, which provides management and monitoring capabilities, is powered on.

References:

Dell XtremIO Deployment Services

Dell XtremIO Deployment Guide and official documentation for detailed step-by-step procedures.