Ever make a new viewport and realize that another one has suddenly gone blank? And when you try to turn it back on, a different one goes blank?
This system variable, stored in each drawing, controls the number of viewports in a layout that can be displayed at one time. These days, the default value is 64, but it used to be 16, and since it’s a per-drawing setting you might still have a bunch in your library with that setting.
So if MAXACTVP is 16, and you create a 17th viewport in the layout, one of them will have to be turned off.
This does not affect plotting, fortunately, but it’s still a little disconcerting to see those blank viewports floating around. Chances are your computer has enough power to show all the viewports you could possibly want to create, so go ahead – set it to 64. Or higher. (But I think it’d have to be a pretty big sheet to get more than 64 viewports on it!)