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Slots are the other sub-features of the Orbment gameplay in the Trails Universe. They are the open spaces in the Orbment where the Quartz are inserted by choice. When a slot is opened, a party member's maximum EP will increase.
At each game's beginning, most of these are closed by default and can only be opened by enough Sepith at the Orbal Factories. Depending on a character's element of aptitude, the cost of opening/upgrading those slots that match the element cost more. (For example, Kloe's element is water and her orbment has two slots for only insert water Quartz, indicated by the color of the slot's outline.)
In Trails of Cold Steel II, all slots are unlocked by default while requiring Sepith to upgrade them, to allow insertion of more powerful Quartz, especially the Lost Arts Quartzes.