Identifies or enumerates a particular instance as specified by the user.
For doors, this property enumerates instances within a category by increasing the value by 1 for each instance placed. For example, the first door you place in a project will have a Mark value of 1 by default. The next door you place, regardless of type, will have a Mark value of 2. If you change this value to one that is already used by another door, Revit warns you, but allows you to continue using it. The Mark property of the next door you place will then be assigned the next highest unused number.