Fairing Aero is the smallest fairing in the game (If you don't include the premium Draco Fairing). It used to cover the stage below it. As of game version 1.8.0, only the Explosive Module and the end of a Resupply Module are small enough to fit under this tiny fairing, thus it is primarily used to cover rocket stages or small station parts directly to prevent overheating during launch.

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Aero fairing covering a Space Station module