You use the physical and usable capacity of the drives you employ in your storage systems to ensure that your storage architecture conforms to the overall system capacity limits and the size limits of your aggregates.
To maintain compatibility across different brands of drives, Data ONTAP rounds down (right-sizes) the amount of space available for user data. In addition, the numerical base used to calculate capacity (base 2 or base 10) also impacts sizing information. For these reasons, it is important to use the correct size measurement, depending on the task you want to accomplish:
Parity, dparity, and cache drives are not counted against the maximum aggregate size.
To see the physical and usable capacity for a specific drive, see the Hardware Universe at hwu.netapp.com.