Slabs

Slabs are half-blocks. Slabs placed on the ceiling occupy the top half of the block space. They can also be used for a long stair case.

Natural Occurrence
Mod slabs do not generate naturally like normal ones do.

Usage
For a long stair case, or for nice decoration.

Wall Slabs
Also called vertical slabs this was added in the snapshot version 15w43a. Take a chisel, sneak and right click a mod slab. It will flip and go vertical. Click it again to change the orientation. This only works on the mod slabs, not the normal ones. You can only do this if the slab is on the bottom half of a block, not on the upper half.

MCE himself uploaded a gif showing the wall slabs.

Trivia

 * Mod slabs work the same as normal slabs.
 * Slabs can be placed on top of each other to make a double slab, or what seems to be a full block.
 * Wooden slabs can burn.