A Pet Mod will usually have a guiding principle ("Make everything OU-viable"), but it will still consist of a long series of specific changes to achieve the vision of the creator or creators. ("Creators" may mean the community, collectively) AbNormal is a good example of a Pet Mod because while the goal of the project is simple -remove the Normal type from the game- there is no simple rule to this. AbNormal would be an OM if it simply said "All instances of Normal typing are replaced by the Fighting type", but since individual Pokemon and moves are converted to a wide variety of types by no clear underlying rule (Beyond "Not allowed to stay Normal"), such that you must read the entire list of changes to have a clear idea of the meta, AbNormal is a Pet Mod.