After the attack on the Jakku village, Finn and the rest of the Stormtroopers return to the Star Destroyer Finalizer. There, he removes his helmet, and Captain Phasma calls him by his designation (FN-2187) without even seeing his face.
How did she know who he was? Are Stormtroopers' designations printed on their uniform? Some sort of HUD in their helmets? Was she really familiar with the back of his head in particular?
