These preferences allow you to determine if the software will allow to create multiple wiring equipment items or just one item under a vacant slot.
To access these preferences, in the Preferences dialog box, in the tree view pane, click 
beside Wiring and then click I/O Card.
Assign slots for racks
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All slots (vacant and occupied) — Allows you to create multiple wiring equipment items or double-width I/O cards under vacant slots.
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Vacant slots only — Allows you to create only one wiring equipment item per vacant slot. The software will not allow you to add a double-width card or another wiring equipment item under an occupied slot.