Multiplayer Lore is lore based explanations given for multiplayer game mechanics often found in official guides or art books.
Background[]
Doom II Official Strategy Guide[]
The Doom II Official Strategy Guide discusses multiplayer from perspective of Marine Sergeant Workman training Marine Troops in the skills of Squad Based Tactics they will be sent into to fight the invading Demons. This is in reference to the four player co-op mode.
Art of Doom[]
Art of Doom contained lore for the multiplayer armor and other features, while linking it to a arms and resource race between competing factions, and forces of Hell.
Ongoing Territorial Disputes over off-world resources in the twenty-second century would ultimately have an effect on interplanetary corporations and organizations seeking to complete in uncharted space. The increase of space piracy and corporate-funded mercenary subterfuge would necessitate a certain combat readiness on behalf of corporations vying for profits. Seeking to integrate itself into ever-growing interplanetary war industry, the UAC would seize the opportunity to supply warring factions in regions ranging from Mars to distant outposts as remote as Neptune's moons. With armaments of battle armor and experimental weaponry considerably more advanced than those of its competitors, the UAC effortlessly define itself as a prominent arms dealer for the new frontier.[1]
Beyond this there is also lore for the additional multiplayer Demons in the book.
Art of Doom Eternal[]
The Art of Doom Eternal presents the multiplayer Battlemode as a interdimensional battle based on his past experiences (ancestral and memetic), apparently across the entire multiverse.[2] Even Player Customization has lore surrounding it based on the idea of the interdimensional histories (and trying to stop 'history from repeating itself').