You use the physical and usable capacity of the disks 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 disks, 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 and dparity disks are not counted against the maximum aggregate size.