Question marks on dimensions

I've been noticing something that I've never seen before, and don't know what it means. When a dimension uses an equation to evaluate it, there is a red question mark on the dimension, like shown in the picture. I've used equations forever and have never noticed this. In the past when a dimension was evaluated with an equation, it used to show the greek symbol. Does anyone know what the question mark means? Did they simply change equation dimensions from the greek symbol to the question mark? I couldn't find anything about it in the help file. But since they've changed the help file a release or two ago, I can rarely find anything I need in it.