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Nameplates in Diablo III refer to the visible health bar at the top of the screen of the selected target. When Diablo III was announced, there was a lengthy gameplay movie shown where the nameplates would hover roughly around where the cursor was. One nameplate would be visible at a time. Due to the screen being as busy as it is, Blizzard decided to make the change to have the nameplate at the top of the screen, in a central position, to help alleviate "noise". Nameplates for bosses are always visible, presumably while they're in range.

Siegebreaker nameplate in 2008.