1st This value must be unique for each element in a project. Revit Architecture warns you if the number is already used but allows you to continue using it. You can see the warning using the Review Warnings command.
2nd When a element is grouped, the mark parameter (along with phasing) is one of parameters that can be changed. This means if you come up with a user made instance parameter for example; door number, it won’t work if the door is grouped.
3rd Some category elements will automatically number the elements as you put them in the model. So as you insert them it will number them sequentially. The Architectural categories that give an actual number to the element include:
It should be noted thought; even though Revit does not actually put a number on the other categories, it still knows the draw/insert order. If you create a schedule and sort it by Mark, despite the cells being blank the sorting order will be correct. So it’s just a mater of putting the numbers in manually.