Posted this in my WIP thread too, but figured the tutorial section is well suited for this too!
I explain some of the uses for this custom depth buffer like material outlines & depth culling of transparent meshes.
link to tutorial: http://www.tomlooman.com/the-many-uses-of-custom-depth-in-unreal-4/
Excerpt from the post:
“Unreal 4 introduced a new depth buffer along with their PBR rendering system. It’s called “Custom Depth” and can be used for effects like the selection outline that is built-in the Editor, rendering of occluded meshes or custom culling of transparency. I will discuss some of the basics of using this buffer in the material editor and explain some of the features I used it for in the past.”