Room-over-room designates methods allowing to transcend Doom's 2.5D engine. Creating a room above another room is impossible, though the effect can be faked in the normal Doom rendering engine through various tricks (such as moving sectors, as in Industrial Zone's south-east building, self-referencing sectors, etc.) or through source port features, notably 3D middle textures, 3D floors and portals; with only the latter two offering a fully convincing effect.
The earliest implementation of room-over-room, provided by the RORDoom source port in 2000, allowed sectors to overlap. This approach is not generally used now.
Doom 64 simulates a ROR effect by changing the floor and ceiling height depending from the direction the player is coming from. This effect is noticeable while using a no clipping cheat since the sectors won't change properly. Theorically multiplayer would break that effect, however Doom 64 has no support for multiplayer.