This is where you can equip such items as Shields or a second weapon if you have the Dual Wield ability. Grips may also be equipped in the sub slot when using a two-handed weapon.

See Shields and one-handed Weapons for the different equipment that can go here.

Sub Slot
This article uses material from the "Sub" article on FFXIclopedia and is licensed under the CC-BY-SA License.