Only Deflectors, Impulse Engines and Shields that modify the appearance of your ship can be equipped in the Visuals tab.

For example, if you like the look of the Romulan Shields, but like the stats on the Nukara Shields, you will be able to get the best of both by equipping the Nukara Shields to your ship, and the Romulan Shields to your new Visuals tab. This will let Captains use the equipment that suits their playstyle, while simultaneously allowing them to customize their ships appearance with the equipment that suits their tastes. The items equipped in the slots in the Visuals tab overrides the appearance of any matching item that is normally equipped. Captains will now be able to equip Deflectors, Impulse Engines and Shields in the new Visuals tab to modify the visual appearance of their ship, without using the stats or set bonuses on those items. With Season 11.5, we’re adding a new and long requested feature from Captains who wanted greater control over the appearance of their ship.

Star trek online shield visuals