Indicate whether the value you specify in the Volume Size field is the total volume size, which includes space reserved for storing Snapshot copies, or only the space usable by the active file system, which does not include the Snapshot reserve.
By default, the Snapshot reserve is 20% of the volume, but you can change the percentage of space reserved for Snapshot copies in the Snapshot Reserve field.
Select the button that corresponds to the volume size type you want to use.