Skin

Skins are cosmetic changes to the appearance of the player character, but without changing gameplay. They are particularly useful in multiplayer modes to allow to easily distinguish between players.

Classic games
The original Doom engine games (Doom, Doom II, Final Doom, Heretic, Hexen, Strife, and so on) did not truly support skins, merely offering palette translations for the player sprites.

A skin support feature first appeared in Doom Legacy with the S_SKIN lump. This was then adopted by ZDoom, and extended by its multiplayer-oriented derived ports such as Skulltag. Other ports, such as Eternity Engine, followed suit. Since skins are a source port feature, all skins are third-party add-ons. The website Epidermis Emporium served as a repository of downloadable skins for Doom.

Doom Eternal
Doom Eternal supports skins for both the Doom Slayer (including during the single-player campaign, where it will be shown during third-person cutscenes as well as on any first-person animation showing the arms and hands of the player character) but also for the playable demons of Battlemode. A few Slayer skins can be unlocked in the Fortress of Doom during the main campaign, but other skins are contingent on joining the Slayers Club and other requirements.