Partial conversion
From DoomWiki.org
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, a partial conversion retains enough visuals, sounds, or gameplay mechanics to be considered a variant of the original, not a separate game. Thus, megawads such as Obituary and STRAIN are often referred to as partial conversions, as opposed to projects such as Batman Doom or Aliens TC that overhaul the game much more heavily. Nonetheless, the difference between partial and total conversion can be quite loose, leading to some projects (including STRAIN itself) being referred to as both depending on the circumstance.
