The "Members default disk quotas" tab present one sub-tab for each disk device with user quota enabled. These tabs controls the default disk quotas for each member of this group. Most fields are optionals. The effective value for a user is inherited from the defaults associated with the primary group of the user and the defaults for the device. You only enter the value in the area you wish to override.
Do not mix this with group quotas. These are defaults applied separatly to each user, member of this group. This is applied to the user disk quotas settings.