How can I use new half walls?

I have a question, and I don't really know where to post but if someone could advise me, that would be awesome. In the build mode concerning the half wall, they added two taller half/partial walls. However, how do you use them? They are grayed out where I can't select them and if I put the cursor over them it tells me ,"Requires a taller level height" What does that mean and how do I use them?