Terrainregion Attribute Type

What is this for? And how do I use it.

It represents a terrain region, just like you have attributes for regular regions. You use it like any other attribute, even though you probably can't do much with it in DM.

Now that you mention it, what exactly can you do with a Terrain (Region) in DM?

I may be missing something, but a quick overview of the blocks shows that you can do... nothing.

I don't think I ever created blocks for design mode.  Although it can be done by mirroring those of the regular regions. 

Are blocks planned post-launch maybe?

Likely no, unless the feature sees a lot of uptake and need for it. It's same same reason why there are no joint blocks. Most of the time, you don't modify joints after creating them.