The two levels of data an element carries - shared by every copy, or unique to one. One of the first genuinely confusing ideas for anyone new to Revit. A Type parameter is shared by every placed instance of that type - change the fire rating on one door of "Door Single 900mm" and every door of that exact type updates, because it is really one shared definition. An Instance parameter belongs to one specific placed element only - its Mark number, its Level, its exact height above the floor - and changing it affects nothing else.

Getting this distinction right is the difference between a model where a single correction fixes forty doors at once, and one where you are manually fixing forty doors one by one.