Partial conversion

A partial conversion is a PWAD that replaces a portion of the resources of the original game's IWAD. In contrast to a total conversion, the partial retains enough visuals, sounds, or gameplay mechanics to be considered a variant of the original, not a separate game.

One popular type of partial conversion is called a gameplay mod. Many of these have been created for classic Doom, replacing the weapons or monsters or specific gameplay mechanics, while preserving the rest of the original game. Thus it's common for players of mod-friendly source ports, like GZDoom, to combine multiple gameplay mods in a cohesive, non-overlapping manner.