So a bit of a one-off use case, but could be helpful for other applications.... here we go.
I am tasked with creating a retainment wall height schedule of as part of a development application requested by the AHJ. The base of the retaining walls are below the site grade (obviously) and "height" is defined as "T.O. Wall to the T.O. low grade abutting the retaining wall. This site is steeply sloped, so there a of walls with variable heights and grades....
My work around to great a revit schedule would be if I could set a "Mark" instance parameter to labels retaining wall "a" and it corresponds to low-grade "a", etc. in a dedicated spot elevation marker to then filter each top of wall with its corresponding grade-low for each wall. Unfortunately, Revit doesn't allow Mark values on spot elevations and you cannot "edit" system families.
Again, I'm sure there would be wider applications than my particular use-case above where you would need to add a Mark to a spot elevation and use it for scheduling and of filtering purposes. Odd that a Mark Instance is a default parameter built into most other components of this software....
Anyways, wouldn't it be nice....