Realms of Sorcery defines non-true Dhar as accidental creation and use of Dhar by untrained casters allowing for powerful non-Wind spells, or its deliberate creation as part of other Lores like Necromancy. It seems like this is its way of explaining Witchcraft and Hedge Wizards being able to cast spells that are non-Wind but also not explicitly evil magic-y, and though this is a neat solution, it's contradicted by things like the Lore of Hags and the Lore of Hedgecraft being introduced in other 2e books. But it says that while ambient stagnant magical energy is 'True Dhar', actually taking that magic and using it for Necromancy, or Skaven magic, or even Chaos Sorcery would not be an application of True Dhar, it becomes downgraded to only regular Dhar if it's used that way. It's only True Dhar if it's used for Dark Magic, which happens to be used only by Dark Elves. Hence my suspicion is that this is a differentiation invented by Dark Elves.