Illumination
Light is the substance that falls from the sky and lights up blocks. In Creative and Survival, land falls upon any surface that is completely uncovered, a single block separating the block from the sky causes darkness. The the playtest, light is much more dynamic, allowing it to fall down "levels" which there are nine of.
PlayTest Light Dynamics
Light in the indev works by checking all adjacent tiles to see if they have a lower light value then itself. If so, it will raise it to 1 below itself.